Board Thread:Dark Souls II/@comment-207.191.188.83-20140317123556/@comment-24700697-20140330074225

The bosses will respawn with the use of a bonfire ascetic, however you must use the ascetic at the appropriate bonfire. usually this is the last bonfire you encounter before fighting a boss. For instance, the hidden chamber bonfire in Black Gulch will respawn The Rotten. The bonfire in the secret room above Mytha's arena will respawn her, and the bonfire right before the Iron Keep boss will respawn him.