The Eyes of the Dead is a legendary crafting material used to craft a 2-H weapon. The Eyes of the Dead is a random drop from Headcleaver in the campaign mode. He is the skeleton that spawns when Deckard Cain is under attack at Leoric’s Passage in the Tristram Cathedral. Set your game to start at The Legacy of Cain: Explore Cellar.
In Adventure-mode, there is a Bounty: Kill Merrium Skullthorn in Act I: Tristram Cathedral level 2. Spent about four hours on this one. Not recommended. On top of the random chance at looting Eyes of the Dead from him, there is the randomness of the bounty spawning him. The bounty won’t be available every time you create an adventure-mode map.