From a Throne of Skulls

Hidden agenda (Start the game with this conspiracy
face down in the command zone. You may turn this
conspiracy face up at any time)
You may only flip From a Throne of Skulls face
up on your turn. If a creature you don't control
died this turn and no other creatures are on the
battlefield you may flip From a Throne of Skulls
face up. If you do you become the monarch.