Tragic Slip

Target creature gets -1/-1 until end of turn.
Morbid — That creature gets -13/-13 until end of turn instead if a creature died this turn.

Swamp

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.)

Unbreathing Horde

Unbreathing Horde enters with a +1/+1 counter on it for each other Zombie you control and each Zombie card in your graveyard.
If Unbreathing Horde would be dealt damage, prevent that damage and remove a +1/+1 counter from it.

Ghoulcaller's Chant

Choose one —
• Return target creature card from your graveyard to your hand.
• Return two target Zombie cards from your graveyard to your hand.

Fleshbag Marauder

When Fleshbag Marauder enters, each player sacrifices a creature of their choice.

Liliana, Death's Majesty

+1: Create a 2/2 black Zombie creature token. Mill two cards.
−3: Return target creature card from your graveyard to the battlefield. That creature is a black Zombie in addition to its other colors and types.
−7: Destroy all non-Zombie creatures.

Lord of the Accursed

Other Zombies you control get +1/+1.
{1}{B}, {T}: All Zombies gain menace until end of turn.

Skinrender

When Skinrender enters, put three -1/-1 counters on target creature.

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.

Wight of Precinct Six

Wight of Precinct Six gets +1/+1 for each creature card in your opponents' graveyards.

Murder

Undead Warchief

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

Endless Ranks of the Dead

At the beginning of your upkeep, create X 2/2 black Zombie creature tokens, where X is half the number of Zombies you control, rounded down.