Dagon Greyjoy, called the Last Reaver, was Lord of the Iron Islands and Lord Reaper of Pyke during the reign of King Aerys I Targaryen. He last sat in the Seastone Chair "almost a hundred years" before the events of A Song of Ice and Fire.
By 211 AC, Dagon and his ironborn were raiding along the Sunset Sea and the western coast of Westeros. They raided as far south as the Arbor, attacking and burning Little Dosk and carrying off half the wealth of Fair Isle and a hundred women. Dagon sent ironborn north, where Lord Beron Stark was gathering swords to drive him off, and Lord Tybolt Lannister was building ships to fight him on the seas. Dagon retreated to Pyke whenever his adversaries retaliated. Erik Ironmaker sailed with Dagon, who was eventually defeated by House Targaryen.
Lord Dagon and Dagon's Feast, longships of the Iron Fleet, are named after Dagon.
In Dagon's day a weak king sat the Iron Throne, his rheumy eyes fixed across the narrow sea where bastards and exiles plotted rebellion. So forth from Pyke Lord Dagon sailed, to make the Sunset Sea his own. He bearded the lion in his den and tied the direwolf's tail in knots, but even Dagon could not defeat the dragons.