Rhatigan is a Carta Boss assigned to capture Hawke.
Background
Rhatigan is the leader of the Carta surface dwarves in Kirkwall, operating out of his headquarters in the Vimmark Mountains.
Rhatigan, along with his comrades in Vimmark, have been driven insane from eating darkspawn flesh as a deranged ritual to better attune to their dark master's will, whom they seek to free from his prison—an odd desire for the Carta, whose members pride themselves on business and brutality.
It is revealed that the Grey Warden Janeka had contracted the Carta under Rhatigan to capture Hawke and assist as her enforcers in the prison tower.
Involvement
Rhatigan is the last boss of the Carta Hideout in the Legacy DLC during Legacy (quest). He fights alongside his bronto Wanka and several snipers.
Result
Killing him will have Hawke equip The Hawke's Key.