Zombie Apocalypse

Return all Zombie creature cards from your graveyard to the battlefield tapped, then destroy all Humans.

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.

Empty the Laboratory

Sacrifice X Zombies, then reveal cards from the top of your library until you reveal a number of Zombie creature cards equal to the number of Zombies sacrificed this way. Put those cards onto the battlefield and the rest on the bottom of your library in a random order.

Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Necroduality

Whenever a nontoken Zombie you control enters, create a token that's a copy of that creature.

Fleshbag Marauder

When Fleshbag Marauder enters, each player sacrifices a creature.

Gleaming Overseer

When Gleaming Overseer enters, amass Zombies 1. (Put a +1/+1 counter on an Army you control. It's also a Zombie. If you don't control an Army, create a 0/0 black Zombie Army creature token first.)
Zombie tokens you control have hexproof and menace.

Diregraf Captain

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

Midnight Reaper

Whenever a nontoken creature you control dies, Midnight Reaper deals 1 damage to you and you draw a card.

Cleaver Skaab

{3}, {T}, Sacrifice another Zombie: Create two tokens that are copies of the sacrificed creature.

Rooftop Storm

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

Death Baron

Skeletons you control and other Zombies you control get +1/+1 and have deathtouch.