Headcleaver, Royal Executioner, is a super unique Royal Henchman in Act I of Diablo III, found in Leoric's Passage. He was tasked with beheading Deckard Cain, and must be killed for the Legacy of Cain quest to be completed.
He attacks with multiple assistant skeletons, and can summon more every few seconds. His own affixes are Extra Health, Fast and Teleporter, and in addition he can kick the player instead of attacking normally.
There is an Cut Off the Head achievement for killing him before any of his minions are killed.