If you attempt to hand in the detonator to one of the Alters and the room is not clear, the alter will eat the detonator and you will need to restart the mission. Not sure if this is a bug, or "working as intended" but be sure to clear the rooms. It seems each Alter room has 3-4 mobs in it. Side note: I cleared all the priests in the zone and did not get the achievement, so this one might be a bit buggy.