Have

Gravecrawler

Gravecrawler can't block.
You may cast Gravecrawler from your graveyard as long as you control a Zombie.

Death Baron

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

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.)

Unholy Grotto

{T}: Add {C}.
{B}, {T}: Put target Zombie card from your graveyard on top of your library.

Scourge of Nel Toth

Flying
You may cast Scourge of Nel Toth from your graveyard by paying {B}{B} and sacrificing two creatures rather than paying its mana cost.

Cabal Coffers

{2}, {T}: Add {B} for each Swamp you control.

Undead Warchief

Zombie spells you cast cost {1} less to cast.
Zombie creatures you control get +2/+1.

Hero's Downfall

Swamp

Grave Pact

Whenever a creature you control dies, each other player sacrifices a creature.

Entomb

Search your library for a card, put that card into your graveyard, then shuffle.

Carrion Feeder

Carrion Feeder can't block.
Sacrifice a creature: Put a +1/+1 counter on Carrion Feeder.

Accursed Centaur

When Accursed Centaur enters, sacrifice a creature.

Corpse Harvester

{1}{B}, {T}, Sacrifice a creature: Search your library for a Zombie card and a Swamp card, reveal them, put them into your hand, then shuffle.

Dark Ritual

Skullclamp

Equipped creature gets +1/-1.
Whenever equipped creature dies, draw two cards.
Equip {1}

Relentless Dead

Menace (This creature can't be blocked except by two or more creatures.)
When Relentless Dead dies, you may pay {B}. If you do, return it to its owner's hand.
When Relentless Dead dies, you may pay {X}. If you do, return another target Zombie creature card with mana value X from your graveyard to the battlefield.