Rooftop Storm

You may pay {0} rather than pay the mana cost for Zombie creature spells you cast.

Wayward Servant

Whenever another Zombie you control enters, each opponent loses 1 life and you gain 1 life.

Wilhelt, the Rotcleaver

Whenever another Zombie you control dies, if it didn't have decayed, create a 2/2 black Zombie creature token with decayed. (It can't block. When it attacks, sacrifice it at end of combat.)
At the beginning of your end step, you may sacrifice a Zombie. If you do, draw a card.

Diregraf Colossus

Diregraf Colossus enters with a +1/+1 counter on it for each Zombie card in your graveyard.
Whenever you cast a Zombie spell, create a tapped 2/2 black Zombie creature token.

Death Baron

Skeletons you control and other Zombies you control get +1/+1 and have deathtouch. (Any amount of damage they deal to a creature is enough to destroy it.)

Diregraf Captain

Deathtouch
Other Zombie creatures you control get +1/+1.
Whenever another Zombie you control dies, target opponent loses 1 life.