O-Shiadan, of the Grimskull

{1}{b}: Summon a 1/2 Grimskull Zombie creature token, but as an additional cost, exile a creature from your graveyard.
When O-Shiadan, of the Grimskull is summoned, summon 4 1/2 Grimskull Zombie creature for each creature in your graveyard.
Each time a creature with the Zombie sub-type destroys an enemy creature, summon a 1/2 Grimskull Zombie creature token.