Wildfire

"Piss on wildfire, and your cock burns off."

- Tyrion Lannister quoting a sailor's proverb

Wildfire is a dangerous liquid which is created and controlled by the Alchemists' Guild, an ancient society of learned men boasting arcane knowledge now based in the city of King's Landing. It is a highly volatile material which can explode with tremendous force and burns with a fire that water cannot extinguish, only large quantities of sand.

Wildfire is identifiable through the distinctive green hue of its flames and a bright green color in its liquid state. Pyromancers refer to it as "the Substance", while in the lowest circles it is derisively referred to as pyromancer's piss.

Wildfire played a key role at the Battle of the Blackwater, during the War of the Five Kings.

Season 2
Thanks to his reluctant agent, Lancel Lannister, Tyrion is alerted to Cersei's plan to use wildfire in the defense of King's Landing against the impending attack by Stannis Baratheon, throwing it from the walls with catapults. Wisdom Hallyne of the Alchemists' Guild shows Tyrion their stocks. Bronn derisively says he doesn't believe the pyromancers' claims about wildfire, as there are always charlatans on the side of battlefields selling "pig shit" to soldiers and claiming it has magical properties. Hallyne insists that the abilities of wildfire are indeed very potent. Bronn then points out that even if this is true, wildfire is too unstable to be used safely in such a manner by inexperienced troops: the moment one of the catapult teams drops a single jar, it could burn the whole city to the ground by accident. Hallyne then opens the guild's vault to reveal to an awed Tyrion that they have amassed over 7,800 jars of wildfire, enough "to lay King's Landing low". Tyrion agrees and commands Hallyne to begin producing wildfire for his purposes instead.

While discussing the defense of the city with Varys and Bronn, Tyrion identifies the Mud Gate as the city's most vulnerable spot and the most likely location for Stannis to attempt a landing. Tyrion resolves to make a stand there. When Varys asks him what can they use to defend the city, Tyrion replies "pig shit".

When Stannis Baratheon's fleet arrives at Blackwater Bay, Ser Davos Seaworth (commanding the vanguard of the assault) is bemused to see only one ship from the royal fleet emerge to challenge them. He realizes the ship is unmanned so he halts his archers, but then sees the ship is emptying wildfire into the bay from pipes at its rear. Tyrion loaded all eight thousand jars of wildfire into the ship. At Tyrion's command, Bronn fires a burning arrow into the wildfire, which detonates the ship in a massive explosion that pulverizes all of Stannis' ships closest to the blast. The spreading fire then engulfs nearby ships. Hundreds, if not thousands, of sailors and soldiers under Stannis's command are burned alive. Stannis resolves to continue the assault regardless of the losses of ships and men.

Season 3
While touring the Great Sept of Baelor with Margaery Tyrell, King Joffrey Baratheon points out an urn containing the ashes of Prince Aerion Targaryen who died drinking wildfire, believing that the substance would turn him into a dragon (Aerion was wrong).

While convalescing at Harrenhal, Jaime Lannister reveals to Brienne of Tarth that the real reason he killed Mad King Aerys II Targaryen is because he was planning on burning the entire capital city to ashes with wildfire, rather than let it fall to the rebels. When it began to appear that the rebels would win the war, the Mad King had his pyromancers hide caches of wildfire all over King's Landing - under the Great Sept of Baelor, under the slums of Flea Bottom, under houses, stables, taverns, and even under the Red Keep itself. Tywin Lannister arrived at the capital city at the head of a Lannister army, before the rebels, but as soon as the Lannister army was inside the gates they betrayed the Targaryens and proceeded to sack the city. The Mad King was going to burn the entire city and all of its half a million inhabitants, convinced (somewhat like Aerion Brightflame) that he would burn along with everyone else, but that the flames would transmute him into a dragon to destroy his enemies. After King Aerys gave this suicidal order, Jaime realized he could not allow it to happen. First, he killed the pyromancer, and then he killed the Mad King in front of the Iron Throne itself, to make sure that his Wildfire plot was never implemented. Jaime subsequently told no one, thinking that Eddard Stark would never believe him, and no one knew that he had in fact saved the city.

Season 6
Wildfire featured prominently in Bran Stark's visions after his and Meera Reed's escape from the Battle at the Cave of the Three-eyed raven that included visions of dragons and White Walkers.

The exploding wildfire in Bran's vision is not from the days of the Mad King, but instead is actually used later. Wildfire is used by Cersei Lannister and Qyburn to Destruction of the Great Sept of Baelor the Great Sept of Baelor just after the trial of Ser Loras Tyrell. The explosion kills all of those inside and surrounding the sept, including the High Sparrow and the entirety of House Tyrell, with the exception of Lady Olenna Tyrell, who fled King's Landing. As a result, King Tommen I commits suicide, which later allows for Cersei to be crowned Queen of the Andals and the First Men by Qyburn.

Behind the scenes

 * In the German dub, "Wildfire" was renamed to "Seefeuer" which means "sea fire", one of the Byzantine names for Greek Fire. Wildfire (and for that matter, Greek Fire) aren't exclusively used in naval combat, though they are very effective against wooden ships.

In the books
In the A Song of Ice and Fire novels, wildfire is a very unstable material similar to real life napalm or Greek fire. In real life, "Greek fire" was actually used by the Byzantine Empire in medieval times, and thus was actually a technology known to medieval peoples.

Wildfire's use under controlled conditions was popular with some of the members of House Targaryen and some worshipers of the eastern god of fire, the Lord of Light, such as Thoros of Myr, who used to coat his sword with a thin layer of wildfire and then set it ablaze to scare his enemies in combat.

The alchemists refuse to divulge how wildfire is created, claiming that it is done with magical spells (a claim dismissed by others, particularly the maesters). Wildfire is stored in clay pots in bare stone cells under the Alchemists' Guildhall in King's Landing. Each cell is built under a chamber full of sand, which can be emptied into the cell below to extinguish the fire in case of accidents. Wildfire grows more volatile with age and can only be disposed of through hazardous means. Wildfire is highly combustible and can be set alight even by bright sunlight. It is described as having an emerald green coloring once ablaze, which is so strong that it can turn the air around it green.