Two known spawn locations:
Hits for a max ~4,800.
On Aggro: Skeleton Keymaster says 'So, you wish to find the entrance to the next world. I can oblige you.'
On Dispell: Skeleton Keymaster says "Oppression takes many forms. To be free one must leave everything behind, especially those things they value most."
On Death: Skeleton Keymaster's corpse rattles to the ground.