|Although this article is based on canonical information, the actual name of this subject is pure conjecture.
The assassination of Balon Greyjoy is an event during the War of the Five Kings which sees the death of Balon Greyjoy, King of the Iron Islands, at the hands of his own younger brother, Euron Greyjoy.
Several years before the events of the series, Balon Greyjoy exiles the eldest of his younger brothers, Euron Greyjoy. Along the years, Euron establishes himself as a feared pirate captain throughout Westeros and Essos.
After Yara Greyjoy informs Balon that House Glover has retaken Deepwood Motte and they have no more holdings mainland, Balon states that he does not care and intends to send more ironborn to raid and hold the North. However, as he walks onto a bridge between two towers of Pyke during a storm, he encounters Euron, who has returned to the Iron Islands following the deaths of Tywin Lannister and Stannis Baratheon. After a brief exchange in which Euron makes it clear he has come to replace his brother, Balon draws a knife and slashes Euron across the face, but Euron retaliates by wrestling Balon over the bridge, sending him falling to his death.
A funeral is held for Balon, where Yara vows to avenge her father, obviously expecting to succeed him. Her youngest uncle, Aeron "Damphair" Greyjoy, however, informs her that there will be a Kingsmoot to decide who will succeed Balon as King of the Iron Islands.
- With the death of King Balon Greyjoy, all five of the titular kings of the War of the Five Kings (Balon, Robb Stark, Joffrey Baratheon, Stannis Baratheon, and Renly Baratheon) are dead, though the war itself continues between several of the kings' successors, such as Joffrey's successor, Tommen Baratheon; Jon Snow and Sansa Stark, who plan to overthrow the Boltons in the North who betrayed Robb Stark; Brynden Tully who has retaken Riverrun from the Freys and holds it in Robb Stark's name; and Euron Greyjoy, Balon's successor.
- With Balon's death, all three "usurpers" against whom Stannis and Melisandre performed the leech-blood ritual for, using the blood of Robert's bastard son Gendry (Balon, Robb and Joffrey) - are dead.
In the books
While the murders of Robb and Joffrey take place "on-screen" and their murderers are revealed to the readers, Balon's death takes place "off-screen" and its circumstances remain obscure by the point the books reached. The official version is that he was crossing a bridge during a storm and it was broken by the wind; he washed up two days later, all bloated and broken, crabs ate his eyes.
There are, however, hints that it is Euron's handiwork:
- The prophecy of an old woman, known as the ghost of High Heart: "I dreamt of a man without a face, waiting on a bridge that swayed and swung. On his shoulder perched a drowned crow with seaweed hanging from his wings" - implying that Euron hired a assassin from the Faceless Men to kill his elder brother.
- Euron, who was exiled several years ago by Balon, returns to the Iron Islands one day after Balon's death. Asha, more intelligent and analytical than most ironborn, suspects Euron of killing her father and confronts him in public. He dismisses her accusation nonchalantly, claiming he was on his ship, whose crew members are - very conveniently - mutes.
In the recently released sample chapter "The Forsaken", it is finally confirmed that Euron murdered Balon (and two more of his brothers - Robin and Harlon). Euron admits arrogantly to Aeron: "Oh, and Balon was the third, but you knew that. I could not do the deed myself, but it was my hand that pushed him off the bridge". In contrast to the show, he does not admit that in public.
Except Asha, no one has suggested Balon was murdered, accused Euron (or anyone else) of Balon's death, or bothered to investigate it. Nearly everyone at the Iron Islands and Westeros in general, including Aeron and Victarion, simply accept Balon's death as a fact, and the reason does not matter to them.
Neither Asha nor anyone else declares a vendetta against the unknown murderer.
Of note, it isn't clear whether the leech ritual had any effect at all in respect of any of the deaths of the three "usurpers": while it is not clear who or what caused Balon's death, the murders of Robb and Joffrey were obviously not caused by magical means. Davos and Melisandre similarly debate this in the TV series when Robb Stark is killed at the Red Wedding: Davos points out that Robb was killed by the Freys and Boltons for political reasons, not magical means, but Melisandre insists that the Freys and Boltons were the instruments by which the Lord of Light acted - leaving it open to interpretation.
With Balon's death, by the point the books reached Stannis is the only survivor of the five kings, after whom the War of the Five Kings is titled.
Balon's death has a very little effect on the events at the mainland, particulary the War of Five Kings: the only notable result is that Victarion returns to the Iron Islands, leaving behind a skeletal garrison in Moat Cailin, what would have made it easier for Robb Stark to liberate Moat Cailin had he not been killed in the Red Wedding - and made it easier for Theon to later persuade the ironborn to surrender to the Boltons.
In the TV series, just prior to the Red Wedding Robb Stark was preparing for a desperation strike against Casterly Rock because it was the only place his enemies' armies weren't concentrated in force, and he only returned to the Twins to reconcile with the Freys and unite with their local army again. This was apparently done as a "hope spot" to make the audience think Robb Still had some chance of turning the war around, prior to the surprise of the Red Wedding.
In the books, Robb actually intended to march his entire army back into the North to drive out the ironborn and reconsolidate his remaining powerbase - even though this would leave his Riverlands allies badly outnumbered (Robb left Brynden Tully in charge of the southern front in the hope that his experienced generalship would be able to hold the line for a time). Catelyn Stark desperately tries to warn her son that with the defeat of the Baratheons after the Blackwater the war is clearly lost, the Lannisters can afford to slowly grind them down through attrition on a scale of years, and they have no realistic hope of winning anymore. For that matter, driving Balon's ironborn from the west coasts will be no easy task, and Robb's armies might be stuck fighting on two fronts for years. Morever, since Ned, Bran and Rickon are dead, Sansa is married to Tyrion and Arya is lost - Robb is the only Stark left, and if he dies too - everything will be lost for the North. She begs him to bend his knee to the Lannisters, claiming there is no shame in yielding, as Torrhen Stark did to Aegon the Conquerer long ago; the Lannisters do not need the north, all they will demand is homage and hostages (and they already have one - Sansa). Robb asks his mother bluntly "Did Aegon kill King Torrhen's father?" and stubbornly refuses, saying he will never surrender to those who honorlessly killed his father, and he will bleed every Lannister army sent into his home territories using the defender's advantage.
News of Balon's death only reaches Robb as his army is retreating back to north to the Twins, as they pass Seagard: the captain of Myraham, who brought Theon to the Iron Islands and was delayed there at Balon's command for half a year, sails back to Seagard, where he informs Lord Jason Mallister the news. Later he tells that to Robb and Catelyn, mentioning also that Euron has returned.
Balon's death is predicted to send the ironborn into chaos - not only for losing Balon's leadership but because they will start fighting each other in a succession crisis, and even pull forces out of the North to fight over the Iron Islands themselves. For once this is a piece of very good news, which does give Robb a glimmer of hope: he might be able to very quickly drive the remaining ironborn out of the North, then return his whole army back south to Riverrun before the Lannisters can mount a new offensive against the Tullys.
Thus, in both versions, when Robb Stark arrives at the Twins the war isn't going well but he has a new plan which might turn it back to his favor - which gets subverted when the massacre of the Red Wedding begins. The TV series probably changed this around to Robb deciding that Casterly Rock was relatively undefended because it would have used up a significant amount of screentime in Season 3 to explain when and how Balon Greyjoy died.
Theon has no idea about his father's death - not that it would matter to him: at the point Balon died, he was imprisoned at Dreadfort, and had much more urgent concerns than pondering about his inheritance rights.