Yorick, The Gravedigger

When Yorick is played onto the battlefield
summon {x} 1/1 Mist walkers
where {x} is the number of mana
u have used to summon Yorick
When an opponent's creature dies
(Graveyard or exile) you may pay {b} to turn it
into a 1/1 mist walker under your control
— "Arise and return to this world."