Have

Dark Ritual

Attrition

{B}, Sacrifice a creature: Destroy target nonblack creature.

Body Snatcher

When Body Snatcher enters, exile it unless you discard a creature card.
When Body Snatcher dies, exile it and return target creature card from your graveyard to the battlefield.

Spawning Pool

Spawning Pool enters tapped.
{T}: Add {B}.
{1}{B}: Spawning Pool becomes a 1/1 black Skeleton creature with "{B}: Regenerate this creature" until end of turn. It's still a land. (If it regenerates, the next time it would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)

Unnerve

Sicken

Enchant creature
Enchanted creature gets -1/-1.
Cycling {2} ({2}, Discard this card: Draw a card.)

Soul Feast

Target player loses 4 life and you gain 4 life.

Looming Shade

{B}: Looming Shade gets +1/+1 until end of turn.

Plague Dogs

When Plague Dogs dies, all creatures get -1/-1 until end of turn.
{2}, Sacrifice Plague Dogs: Draw a card.

Squirming Mass

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)

Scent of Nightshade

Reveal any number of black cards in your hand. Target creature gets -X/-X until end of turn, where X is the number of cards revealed this way.

Ostracize

Target opponent reveals their hand. You choose a creature card from it. That player discards that card.

Swamp

Ravenous Rats

When Ravenous Rats enters, target opponent discards a card.

Bone Shredder

Flying
Echo {2}{B} (At the beginning of your upkeep, if this came under your control since the beginning of your last upkeep, sacrifice it unless you pay its echo cost.)
When Bone Shredder enters, destroy target nonartifact, nonblack creature.

Order of Yawgmoth

Fear (This creature can't be blocked except by artifact creatures and/or black creatures.)
Whenever Order of Yawgmoth deals damage to a player, that player discards a card.

Duress

Target opponent reveals their hand. You choose a noncreature, nonland card from it. That player discards that card.

Nightshade Seer

{2}{B}, {T}: Reveal any number of black cards in your hand. Target creature gets -X/-X until end of turn, where X is the number of cards revealed this way.

Engineered Plague

As Engineered Plague enters, choose a creature type.
All creatures of the chosen type get -1/-1.

Sanguine Guard

First strike
{1}{B}: Regenerate Sanguine Guard.

Dying Wail

Enchant creature
When enchanted creature dies, target player discards two cards.

Disease Carriers

When Disease Carriers dies, target creature gets -2/-2 until end of turn.

Mantis Engine

{2}: Mantis Engine gains flying until end of turn. (It can't be blocked except by creatures with flying or reach.)
{2}: Mantis Engine gains first strike until end of turn. (It deals combat damage before creatures without first strike.)

Slinking Skirge

Flying
{2}, Sacrifice Slinking Skirge: Draw a card.

Polluted Mire

Polluted Mire enters tapped.
{T}: Add {B}.
Cycling {2} ({2}, Discard this card: Draw a card.)

Eradicate

Exile target nonblack creature. Search its controller's graveyard, hand, and library for all cards with the same name as that creature and exile them. Then that player shuffles.

Despondency

Enchant creature
Enchanted creature gets -2/-0.
When Despondency is put into a graveyard from the battlefield, return Despondency to its owner's hand.

Festering Wound

Enchant creature
At the beginning of your upkeep, you may put an infection counter on Festering Wound.
At the beginning of the upkeep of enchanted creature's controller, Festering Wound deals X damage to that player, where X is the number of infection counters on Festering Wound.