The Greyjoy Rebellion
Balon Greyjoy crowned himself King of the Iron Islands and was put down in a season. His last living son, Theon, was taken to Winterfell as a ward ÔÇö which is a polite word for a hostage raised as a brother, and an arrangement that works perfectly right up until the moment it doesn't.
The fuller tale
Six years into Robert's reign, Balon Greyjoy declared the Iron Islands a kingdom again and crowned himself with driftwood. His opening move was to burn the Lannister fleet at anchor in Lannisport, which was well done and which turned out to be the last thing that went right.
The realm answered together, which was the problem: Robert's own host, Tywin's, Ned Stark's, and Stannis Baratheon at sea, where he broke the Iron Fleet off Fair Isle. Then Pyke was stormed. Balon's two eldest sons died in the fighting. Victarion beat the gate down and Thoros of Myr came through the breach first with a flaming sword, which is the image everyone remembers.
Balon knelt. Robert let him keep the islands, on the condition that his last living son came away as a ward ÔÇö a polite word for a hostage. So Theon Greyjoy, ten years old, went north to Winterfell to be raised by the man who had put down his father's rebellion.
A boy in two houses
The arrangement worked for nine years, in the way such arrangements do: Theon grew up with the Stark children, trained with them, hunted with them, and was never quite one of them, and knew perfectly well that his life depended on his father staying quiet.
It stopped working the moment the realm cracked and he was sent home to a father who had spent nine years despising him for surviving. Everything Theon does in the main saga runs out of a rebellion he was too young to fight in.
Elsewhere in the wiki
 See the world as it stood in 289 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.