I started wondering this myself and I do think using the leave faction works, but if you're trying to become a vassal I believe you need 150 renown and you just talk to the leader and it will give you a option to be more than a mercenary.
Khilam the Angel in Iyakis. It's the "Rival Gang Moving In at Iyakis".
Couldn't figure out how to insert a photo.
The enemies spawn on top of a building and slowly fall off leaving one stuck forever.