Once you’ve cleared all the Bandit Camps this side quest will activate. Go talk to Nil, who’ll tell you he’s sad that he has no one else to kill but he wants to meet you in the Spearshafts for one last quest.
Head there and climb up to the Mesa, there you’ll meet Nil, who’ll present you a choice. You can either fight and kill him, or deny him and not kill him. The choice is yours, and the quest will complete either way. If you choose to fight, Nil fights much like any other Elite enemy and shouldn’t pose you many problems by the time you get this quest. If you choose not to, he’ll be disappointed but leave.