Death's Head, also refered as Mothman, is an NPC in UnderMine.
When you talk to it, Death's Head explains that you have reached the end of the game. However, you now can pick up a Summoning Stone. This stone will re-summon all bosses, make the game harder, and you will gather more gold from your adventures.
You will not be able to get the Summoning Stone if you skipped any of the bosses. You will die, and will have to beat those you skipped, and then reach the end again. You don't have to kill all the bosses in the same run, and those you have killed will not respawn (until killing every other boss/es and picking up the Summoning Stone).
Stone of the Recreant will appear instead of the summoning stone if you didn't kill all the bosses.