The Spoils of War

"The Spoils of War" is the upcoming fourth episode of the seventh season of Game of Thrones. It will be the sixty-fourth episode of the series overall. It will premiere on August 6, 2017. It was directed by Matt Shakman.

Plot
Daenerys fights back. Jaime faces an unexpected situation. Arya comes home.

In the North
At Winterfell, Littlefinger meets with the recently-returned Bran Stark. Apparently hoping to exploit disunity amongst the Stark children, he tries to ingratiate himself with Bran - who, as Ned Stark's last trueborn son, should rank ahead of Jon Snow as rightful heir to the North. Littlefinger gives Bran a gift - the very same Valyrian steel dagger, with a dragon-bone hilt, which a cutthroat tried to kill him with while he was in a coma right after he fell from one of Winterfell's towards (actually, was pushed). Bran is still emotionally detached, however, from all of his visions as the new Three-eyed raven. Bran is disinterested, only absent-mindedly asking who the dagger belonged to. With a wry look, Littlefinger responds that in a way that question started the entire War of the Five Kings - Bran's mother Catelyn Stark took the dagger south with her to King's Landing to try to find who it belonged to, convinced the Lannisters sent the cuthroat to kill Bran (and left it there in Ned's possession, from whom Littlefinger recovered it). Littlefinger tries to manipulate Bran by remarking on how much chaos he must have lived through to get back to Winterfell - but then Bran looks at Littlefinger and says "Chaos is a ladder" - quoting back Littlefinger's own words to him, which he said to Varys in King's Landing years before and which Bran couldn't possibly have been physically present to hear himself. Unnerved, Littlefinger takes his leave of Bran, calling him "Lord Stark", though again Bran insists he isn't going to be a lord now.

Meera Reed later comes in to Bran's chamber. Noticing his new wheelchair, he explains Maester Wolkan built it for him. She explains she came to tell him she's leaving and say goodbye: she promised to keep him safe, and now that he's back in Winterfell surrounded by his Stark forces he's as safe as anyone will be before the coming war. She explains that when the White Walkers arrive, she wants to be with her family, so she is heading back to the Neck to assist the Crannogmen. She says he doesn't need her in Winterfell anymore - to which he flatly says he doesn't, and with blank emotional affect simply says "Goodbye". Meera grows angry that this is all he has to say, after everything they went through: Hodor and Summer and even her own brother Jojen died for Bran. He then says that he isn't really, anymore - isn't "Bran Stark" but the three-eyed raven. He "remembers" the events of Bran Stark's life but now "remembers" vast amounts of other accumulated memories from centuries upon centuries. Everything that once affected "Bran Stark" now seems distant and trivial to him. In horror, Meera cries out that he "died in that cave!" - Bran died when the previous three-eyed raven passed on all of the collective memories of generations of greenseers to him.

Meanwhile, Arya Stark finally returns to Winterfell - after leaving years before with her father, Sansa, and King Robert Baratheon's entourage, right after Bran's fall from the tower. Arya rides up to the gates and dismounts but the guards don't believe her when she says she is in fact Arya Stark, convinced that Arya has been dead for years. Arya asks that they send word to Maester Luwin and Rodrik Cassel, who can prove her identity (not knowing they are both dead). The guards brush off that no one by those names is there, so she asks for Jon, but they say he actually just left Winterfell. Arya asks who is in charge of Winterfell then, and they say "Lady Stark" (which Arya realizes is her sister). They try to brush her aside, but she dodges them with her assassin's reflexes, and insists that one way or another she's getting in, and when they try to brush her aside she dodges them with her assassin's reflexes. She explains to them that if she is Arya, they'll be in a lot of trouble for stopping her, and if she isn't, she won't last long in Winterfell anyway. Mildly concerned, they agree to at least let her in the courtyard but insist that she stay put while they send for Sansa so they can disprove her idenity. As soon as they take their eyes off her, however, she slips away. The two guards go to inform Sansa and try to wave it aside as just some imposter, but she instantly realizes it must be Arya, and knows where she has gone.

Sansa finds Arya where she expected, in the crypts looking over their father Ned's grave. They are happy to see each other but so much has happened to both of them in the past few years that they are at first awkward, unsure of what to say. Arya asks if she has to call Sansa "Lady Stark" now, to which Sansa firmly insists "Yes" - and then laughs. They smile and hug, though still a bit unsure. The reunited sisters them look sadly on their father's grave statue. Arya says it doesn't really look like him. Sansa acknowledges that everyone who really knew his face is dead. Arya points out they're not.

Arya then asks that everyone says Sansa killed Joffrey. She explains she actually didn't, though she wished she had. Arya remarks that he was always at the top of her "list". This confuses Sansa, and Arya explains that she'd been keeping a list of everyone she was going to kill - at which they both laugh. Finally Sansa asks how Arya got back, but she only says her road wasn't a pleasant one. Sansa says hers wasn't either. They hug again, earnestly and warmly. Sansa then informs Arya that Bran is home too.

Sansa brings Arya to Branin the Godswood, where he is lost in thought by the Weirwood heart tree. Arya is saddened to see him so paralyzed. Still somewhat detached even at the sight of Arya, he says he isn't surprised she's alive because he saw her at the Crossroads. Arya is confused, and Sansa explains that Bran is having "visions" now. Bran says he thought Arya was going to King's Landing, and when Sansa asks why she would head there of all places, he again startles them both by saying it's because Cersei is on her list of names (which he can't possibly be aware of through normal means). Sansa asks who else is on her list, but she says most of them besides Cersei are actually dead already.

They then remark on the Valyrian steel dagger in his lap, and he explains that Littlefinger gave it to him, thinking he'd want it. Despite it being such a horrible keepsake that nearly killed him and indirectly set off a chain of events leading to his parents' deaths, he is still listless and disinterested in it. Arya is confused: why would a common cuthroat have a rare, priceless blade of Valyrian steel? Bran matter-of-factly says that someone very wealthy wanted him dead, and gave it to the assassin. Sansa acknowledges she doesn't actually trust Littlefinger and he'd never give anyone anything unless expecting something in return.

Bran says that doesn't matter, because he doesn't even want it. Instead, Bran hands it to Arya and says she can have it - because it's "wasted on a cripple". Sansa looks down, dejected at how her brother considers his own physical state to be useless.

All three Stark children, finally reunited, proceed back to Winterfell's castle courtyard, pushing Bran in his wheelchair. Brienne of Tarth and Podrick Payne see them from afar, all three of Catelyn Stark's surviving children reunited. Pod says Catelyn would be proud of Brienne, but she chides that she did next to nothing. Pod says he disagress with "my lady", and Brienne starts to correct him that she doesn't consider herself a "lady", but then stops halfway and just thanks him for the compliment.

Some time later, Brienne is going through a vigorous sword practice session with Podrick, knocing him down when he overextends himself. Impressed, Arya interrupts and says she'd like to spar with Brienne - the woman who beat the Hound in combat. Sansa and Littlefinger watch on silently from the walkway above. Brienne goes easy on her at first, but then Arya completely outmaneuvers Brienne using the Water Dance training she received from Syrio Forel, augmented by her training with the Faceless Men. Arya achives many openings that would be a killing blow if she wanted them to be. Surprised that such a young girl is so skilled, Brienne stops holding back, leading to a grueling sparring session. Brienne actually manages to knock Needle out of Arya's hand but she simply switches to the Valyrian steel dagger she had in her belt. Arya uses her speed and agility to compensate for Brienne's strength and size to overwhelm her - until Brienne actually manages to bring her brute strength to bear on a fast moving target by landing a kick on Arya's chest that sends her falling down. Arya takes her leave of Brienne, both mutually impressed, as Sansa looks down baffled at how her sister received such a deadly skill level.

In King's Landing
In King's Landing, Queen Cersei Lannister is happy to receive all of the gold taken in the Fall of Highgarden. The Lannister army at Highgarden sent the gold ahead to the capital first, though it remains in the north of the Reach seizing grain supplies.

Cersei meets with Tycho Nestoris of the Iron Bank of Braavos in her atrium containing a large floor map of Westeros. Tycho is pleased that Cersei will use the captured gold to pay off the Iron Throne's massive debts to the bank, and he engages in open flattery by saying that she is as cunning at military strategy as her father Tywin was.

Now that the Lannisters' old debts are mostly paid off, and the Iron Bank's faith in them somewhat reassured by their recent military victories, Cersei wants to take out new loans to strengthen her position in the war, so she can finish securing control over the rest of the continent. They discuss that Cersei wants to use the money to hire foreign Sellsword companies to bolster the depleted Lannister military ranks. Specifically, she reveals that she has had Qyburn make overtures to hire the best and largest private mercenary army in all of the Free Cities: the Golden Company.

At Dragonstone
At Dragonstone, Jon Snow and Davos Seaworth wait to meet with Daenerys Targaryen. Davos remarks that Jon seems somewhat attracted to Danerys, but Jon brushes aside that he has seen the Night King and his horde of the undead, and has no time for such concerns. Jon insist that they need to ally with Daenerys, because the North itself is largely depleted of soldiers after so many years of war (and the Red Wedding), so they only have maybe 10,000 soldiers or less left (to which Davos corrects "fewer").

When they encounter Missandei as they wait for her queen to arrive, she politely enquires why Jon's surname is "Snow" even though House Stark has ruled the North for centuries, and his father Ned and brother Robb both had the surname "Stark" but he doesn't. Jon and Davos explain to her the special system regional surnames used in Westeros for bastard children of the nobility. Missandei doesn't know what they mean, so they literally explain that Jon's parents weren't married. Davos asks if they have similar customs for bastards on Naath island where Missandei is from (off the coast of Sothoryos). Quizzically, Missandei explains that "Marriage" as such does not exist in Naath, so she finds the idea of a "bastard" to be quite an alien concept. Davos says that sounds liberating. Daenerys then arrives, and they head down to the beach. Missandei is worried about Grey Worm because they haven't heard any news from Casterly Rock yet. Daenerys wryly asks what happened between her and Grey Worm, and Missandei wrly says "many things". Jon then leads Daenerys on a tour of the long-abandoned Valyrian dragonglass mine on the island (which Samwell Tarly sent him directions to). The mine is ancient, and was used by the First Men long before the Valyrians came to Dragonstone. But this is not why Jon asked Daenerys to come:

As they proceed deeper into the mines led by torchlight, Jon shows her cave paintings they discovered left behind by the Children of the Forest, filled with arcane magical symbols. Daenerys is stunned that the mythical Children stood where they are standing now, thousands upon thousands of years ago, long before mankind ever arrived in Westeros. Jon says this isn't exactly true, leading her into another chamber, this one filled with narrative drawings of the Children and the First Men. Daenerys wonders that the Children and First Men must have fought each other, but then he shows her ancient carvings of the White Walkers themselves, one of which Jon recognizes as an image of the Night King himself. Jon explains that the Children of the Forest and the First Men only managed to drive off the White Walkers the first time during the Long Night because they chose to join together to fight off their own extinction, and that is the choice between Jon and Daenerys now.

Jon still wants Daenerys to ally with him despite not submitting the North to her rule. He insists that the North won't accept a southern ruler again after everything that has happened to them. Daenerys urges that they will if their king does - if his point in showing her the cave drawings was that she should swallow her pride and ally with him because the White Walkers are such a world-ending threat, by the same token, the North should just submit to her, because their survival should be more important than their pride.

As they exit the cave to the beach, they are joined by Varys and Tyrion with grave looks on their faces. They inform her that the Unsullied succeeded in capturing Casterly Rock...only to then reveal the disaster which just occurred as Tyrion's military strategy fell apart. Casterly Rock was only a feint and the Lannisters didn't bother defending it, in order to make a surprise attack south which sacked Highgarden. Daenerys is furious and barely controls her anger, as now all three of her major allies are gone. She blames that Tyrion's gentle strategy lost them Dorne, the Iron Islands (Yara's forces), and now the Reach). She accuses Tyrion that she wonders if he wanted to use a light hand against the Lannisters because they're his own people.  Tyrion insists that they still have just enough ships left to at least shuttle the Dothraki from Dragonstone to the nearby mainland coast:  even with the loss of all her other allies, her combined Unsullied/Dothraki forces are still the largest army in Westeros.  Daenerys fumes that her numbers won't be good for long if she can't feed them all, because Cersei just took all of the food stores from the Reach which they were depending on.  Daenerys decides she has had enough of clever plans, and will use the Dothraki and her dragons in a direct assault to turn the tide of the war back in her favor.

While they are on the beach, they see a lone ironborn ship approaching the island - a survivor of Yara Greyjoy's fleet. Theon Greyjoy himself and some of his men come to shore on a rowboat. As Theon disembarks, he is stunned to see Jon Snow and doesn't know what to say - he hasn't seen him since Jon left Winterfell to join the Night's Watch, long before Theon's own betrayal of Robb and the Winterfell garrison, and before Theon's imprisonment and torture by Ramsay Bolton. Jon is stunned for a moment as well. Theon asks if Sansa is safe, at which Jon walks up and grabs him by his coat and shakes him, angrily saying that what Theon did to safe Sansa from Winterfell is the only reason that he isn't going to kill Theon now. Jon then releases him and leaves, while a dejected Theon silently accepts Jon's reaction.

In the Reach
At Highgarden, Jaime Lannister coordinates the soldiers loading up the spoils of war for transport: the sack of Highgarden allowed them to seize all of House Tyrell's substantial gold stores, which they send ahead to King's Landing first, before moving on to securing grain shipments. Jaime procures a large bag of gold coins and gives it to Bronn as payment for his services. Bronn, however, is annoyed that this isn't the full reward his was promised, which included a wife from the nobility and a castle. He then half-seriously asks why Jaime doesn't just grant him Highgarden, as they need someone to rule it. Jaime waves this aside by saying they don't actually intend to hold Highgarden for long: it would be difficult for Bronn of all people to rule over hostile territory, and the war isn't over yet, so at this point he thinks Bronn should be satisfied with a more movable big sack of gold than a castle he'll have trouble defending. Bronn, however, is not amused.

Some time later, Jaime's Lannister army has advanced much father east along the Roseroad, in the northeast of the Reach. Lord Randyll Tarly comes to Jaime and says they should hurry to get all of their grain wagons over the Blackwater Rush to King's Landing on the northern side, as they will be vulnerable if their formation is caught on both sides of the river. Jaime agrees, but Randyll suggests flogging the stragglers to motivate them. Jaime urges that his soldiers fought well at Highgarden, however, and he should at least give them a fair warning first instead of launching right in to the flogging. Jaime and Bronn then encounter Randyll's son Dickon Tarly again (Samwell's brother). Jaime asks what he thought of his first taste of battle, and he nervously claims it was glorious - only to then dejectedly admit that he was quite conflicted: House Tarly had been loyal vassals of the Tyrells for generations, he knew many of the men they killed, even hunted side by side with them. Jaime earnestly advises the young man that the guilt is not his but Olenna Tyrell's: she was the one who chose to side with Daenerys's foreign army against Queen Cersei, so it is truly Olenna's fault they died - Dickon wasn't involved with the decision making process on either side so he shouldn't dwell on it. Dickon then outright confesses that the aftermath was horrible, how all of the dead bodies stank - Bronn then playfully taunts that Dickon, a sheltered nobleman, finally found out that men void their bowels when they die.

The Lannister army and wagon train continue east across a small river near cliffs and hills (apparently nearTumbleton, see Notes). Jaime and Bronn are then surprised to hear what sounds like distant thunder, until they realize it must be approaching cavalry. Jaime and Randyll shout for their soldiers to form up, which they manage to do before the enemy crests over the horizon - a massive, 100,000 strong horde of Dothraki cavalry. Bronn insists that Jaime should just leave and ride ahead to King's Landing, but Jaime insists he will not abandon his men, and that they have a chance if they hold.

Just then they hear a roar not heard on the battlefields of Westeros in over a century and a half, and look above the Dothraki horde to see see a large adult dragon heading straight for their lines - Drogon, ridden by Daenerys Targaryen herself.

Drogon outpaces the Dothraki, and at Daenerys's command "Dracarys!" he blasts a torrent of fire throat the Lannister ranks in a straight line from front to back, punching a hole in their formation. The highly mobile Dothraki light cavalry immediately sweep through it and wheel around to catch those parts of the Lannister lines in an envelopment.

The Lannister-Tarly lines around Jaime and Randyll manage to rally under their leadership, however: the Dothraki charge into the Lannister lines head-on, but meet stiff resistance as their spear wall holds firm. Though they are outnumbered the Lannister soldiers are better armed heavy infantry, highly disciplined and battle-hardened veterans from years of war. Dothraki horse-archers let off shots before charging into their ranks, but highly trained Tarly archers return fire, doing as much if not more damage as the Dothraki wear less armor.

The Lannister army gives as good as it takes for a time, but it is simply outnumbered by a factor of something like five or ten to one, and the tide turns slowly against them. For a moment it looks like Jaime might at least be able to force the Dothraki to a standstill - but then Daenerys wheels out of the air again, devastating the Lannister formations. No longer trying to punch vertile holes through their lines, she switches to flanking the Lannister ranks, burning a vast swath of men from left to right. Men are flash-burned into nothing but ash which crumbles upon the touch. Men are cooked alive in their own superheated armor as they struggled to rip it off.

Nonetheless, Jaime manages to rally his remaining forcs a second time, desperately taking command of a group of surviving Lannister and Tarly archers and directing them to concentrate their fire at the dragon: Drogon might not be vulnerable to common arror fire, but his rider is. If they can manage to kill Daenerys herself, who has risked appearing in open battle, the entire war could end in a day, no matter their losses. Daenerys sees the attack coming, however, and banks Drogon up so the arrow volley harmlessly bounces off the armored scales on his belly. He then blasts the archer formations with fire. Jaime avoids the flames and is surrounded by enemies, but he manages to carve a path through the Dothraki with his Valyrian steel sword Widow's Wail to try to reach safety.

Even then all is not lost - Jaime also commanded Bronn to reach Qyburn's anti-dragon scorpion-bolt launcher, which they took with them in the wagon train for just this scenario (as Jaime cannot fire it one-handed). Bronn fights his way back, killing several Dothraki until one cuts off his horse's leg and sends him sprawling, losing his large bag of gold in the process. Bronn continues to fight his way back on foot, chased by the Dothraki warrior, who follows him into the wagon he was looking for - just in time to stare down the business-end of the loaded scorpion, which Bronn fires into him, flinging him twenty feet and pinning his corpse to another wagon. Bronn slowly manages to reload the scorpion (which is meant to be crewed by a team of men), spots the dragon in the distance, and fires - but misses. Daenerys is startled by the scorpion-bolt flying a few feet from her head, scans the battlefield, and spots Bronn on the scorpion as he reloads.

Daenerys then leads Drogon on a direct charge across the battlefield at Bronn on the scorpion launcher, who waits for her to approach close enough that he hopefully won't miss but before she is right on top of him. He fires...and manages to hit land a hit in Drogon's right shoulder. This stuns the dragon, making him collapse helplessly into free-fall from hundreds of feet in the air.

The surviving Lannister soldiers cheer - only for Daenerys to manage to shake Drogon back to his senses right before he hits the ground, which would have killed them both. He hovers for a moment and angrily sends a fire-blast at the scorpion, destroying it a matter of seconds after Bronn jumps out of the way to safety.

The dragon's armor scales are so tough that the scorpion bolt didn't manage to penetrate far enough to cause serious internal damage. Nonetheless the flesh wound grounds him, as he howls in pain and destroys any masses of enemy soldiers that get near him. Daenerys sets him down and dismounts to pull the scorpion bolt out.

By this point, the devastation caused by the dragon and the sheer number of Dothraki have totally overwhelmed the Lannister lines, and with the scorpion-launcher destroyed, their last hope is gone and the battle turns into an utter rout and massacre. Tyrion Lannister views the battle from the distance of a nearby hill, along with the Dothraki commander Qhono, who quips that Tyrion's people can't fight. Tyrion looks gravely on the battlefield, realizing that these are in fact his own countrymen from the Westerlands and House Lannister being massacred. Many burned and dying men rush to the river to try to extinguish the flames.

Jaime, however, sees that Daenerys is grounded and immobilized, and realizes all hope is not lost - if he can kill Daenerys, even now, he can end her invasion. Jaime grabs a spear and charges his horse across the flaming battlefield to make a death-run for Daenerys as she tends to her dragon. Tyrion pleads to himself that Jaime is being a fool and going for it.

Just as Jaime is nearly on top of Daenerys, however, the dragon notices him amidst the choas of the battle, shields her behind his head, and points his mouth at him. The dragon lets out a blast of flame, but a moment before Jaime's horse is enveloped, Bronn rushes over to tackle him out of the way. They both fall into the river, where Jaime is weighed down by his armor, and sinks beneath the waters, in danger of drowning.

Cast
Starring
 * Peter Dinklage as Tyrion Lannister
 * Nikolaj Coster-Waldau as Ser Jaime Lannister
 * Lena Headey as Queen Cersei Lannister
 * Emilia Clarke as Queen Daenerys Targaryen
 * Kit Harington as King Jon Snow
 * Aidan Gillen as Lord Petyr Baelish
 * Liam Cunningham as Ser Davos Seaworth
 * Sophie Turner as Princess Sansa Stark
 * Maisie Williams as Princess Arya Stark
 * Nathalie Emmanuel as Missandei
 * Alfie Allen as Prince Theon Greyjoy
 * Gwendoline Christie as Brienne of Tarth
 * Conleth Hill as Varys
 * Jerome Flynn as Ser Bronn

Guest Starring
 * Mark Gatiss as Tycho Nestoris
 * Daniel Portman as Podrick Payne
 * James Faulkner as Lord Randyll Tarly
 * Tom Hopper as Dickon Tarly
 * Brendan Cowell as Harrag

General

 * The episode title is a reference to the spoils seized following the Fall of Highgarden.
 * The Oldtown and Sandor Clegane storylines do not appear in this episode. Arya Stark returns to Winterfell, her storyline merging back in with that one.
 * In a post-episode interview after "Stormborn", writer Bryan Cogman stated that the way the producers conceptualized Season 7, after so many other seasons split amongst half a dozen disparate storylines, is that it is centered around three "axis" points, or sets of characters: King's Landing, Dragonstone, and Winterfell, plus a side-adventure with Samwell at Oldtown.  King's Landing includes Jaime when he's in the Reach, Yara Greyjoy was part of the Dragonstone storyline even in her battle on the Narrow Sea, etc.  The key point is that they used to conceptualize of it as the "Arya storyline", the "Stannis storyline", etc.  In Season 7, Arya doesn't reach Winterfell in this episode, but it's broadly titled as part of the "Winterfell" storyline more than the "Arya storyline" at this point, as it merges back in with that.

In King's Landing

 * The Golden Company is a major subplot from the later books in the A Song of Ice and Fire series, thought abandoned by the TV series. Apparently it will now appear in some condensed form.  Their subplot is filled with major spoilers for the books, which will not be repeated in this article.
 * The Golden Company actually was introduced previously in the live-action TV continuity, exactly once before back in Season 4: after Joffrey died, Davos thought this meant Stannis should make a renewed offensive against the Lannisters, but he scoffed that he had no substantial army left to make such an offensive with.  Davos then suggested that they could then try to hire sellsword armies from the Free Cities, such as the Golden Company, but Stannis equally dismissed that he also didn't have any gold left to hire such expensive mercenaries with.
 * As Davos said in Season 4, the Golden Company is the best and largest sellsword company in the Free Cities, with 10,000 well-equipped and highly trained men - essentially a moderately sized private army for hire.
 * Subsequent installments in the animated Histories & Lore featurettes had Jorah Mormont say that he served in the Golden Company for a time. That hasn't been specifically mentioned in the books, though he did generally say that he drifted between various sellsword companies for several years.
 * In the books, the Golden Company have their own motto of sorts: "Our word is good as gold".  Their war-cry, however, is "Beneath the gold, the bitter steel!" in reference to their founder, a Blackfyre general called "Bittersteel".  They have an excellent reputation and will always honor a contract once signed.  They were founded by losing soldiers from the Blackfyre Rebellion who fled into exile in the Free Cities, augmented over the generations by other sellswords, exiles, etc.  At the current time, the Golden Company consists of about 1,000 cavalry, 1,000 archers, and 8,000 infantry.  About 50 of their crossbowmen are Summer Islanders, whose people are considered the best archers in the known world.  On top of this, the Golden Company prominently fields a number of formidable war elephants.

At Dragonstone

 * The episode trailer reveals Daenerys and Jon Snow inspecting the abandoned Dragonglass mine on Dragonstone. This issue came up in previous episodes:  Stannis told Samwell there was dragonglass on the island back in Season 5, making it unclear why Samwell's discovery in Oldtown's records was such a big revelation.  Reading screenshots of the books Samwell was looking at, however, appear to explain the discrepancy:  there is a long-abandoned main dragonglass mine on the island, built by the ancient Valyrians and used by the First Men before that, which even the Valyrians considered the largest concentration of dragonglass in the known world - literally a mountain of it, beneath the island.  Apparently, the explanation is that Stannis and Davos knew that dragonglass can be found on the volcanic island, just as loose rocks here and there, but even they never knew just how much dragonglass was truly there - the mine was forgotten for generations.

In the books
[This section will be updated with comparisons after the sixth novel is released.]