After the mutilation of King Harmund III Hoare and his mother, Queen Lelia Lannister, at the hands of Hagon Hoare and the priest of the Drowned God referred to as the Shrike, the King of the Rock, Lelia's nephew, declared war on Hagon. Aubrey was a commander of the Lannister troops.
In the seventh year of fighting, Aubrey defeated Hagon and took him prisoner. He considered restoring the rule of Harmund III, who they found in the dungeons of Hoare Castle. Seeing how broken he was, however, Aubrey gave him the gift of mercy. He also ordered that Hoare Castle be razed to the ground. He had Hagon mutilated in the same way that Lelia had been, before having him hanged.
Instead of returning to the Westerlands, however, Aubrey surprisingly named himself the King of the Iron Islands. The Lannisters did not support this decision, and Aubrey's reign lasted less than six months before he was captured. He was then offered up to the Drowned God by the Shrike.