With the aid of a castle servant, Harren seized Harrenhal from Lord Gargon Qoherys, dubbed Gargon the Guest for his tendency to attend every wedding in his dominion and take the right of First Night. Gargon died of bleeding after Harren had him gelded in the Godswood of Harrenhal. Harren then proclaimed himself Lord of Harrenhal and King of the Rivers.
By the time King Aenys I Targaryen and Lord Tully moved against Harren, they found the castle deserted and Gargon's loyal men put to the sword. Harren and his followers then returned to banditry. Aenys worried how to deal with Jonos Arryn when he rebelled, due to not wanting to leave King's Landing vulnerable to Harren.
Harren's men were later routed by a force under the command of the Hand of the King, Lord Alyn Stokeworth. Harren managed to kill Alyn during the fighting, but was, in turn, killed by Alyn's squire, Benard Brune.