Have

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

Ostracize

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

Dying Wail

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

Sicken

Enchant creature
Enchanted creature gets -1/-1.
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.

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.)

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.

Looming Shade

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

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.

Polluted Mire

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

Disease Carriers

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

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.

Plague Dogs

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

Swamp

Duress

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

Squirming Mass

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

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.

Soul Feast

Target player loses 4 life and you gain 4 life.

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.

Sanguine Guard

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

Slinking Skirge

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

Engineered Plague

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

Ravenous Rats

When Ravenous Rats enters, target opponent discards a card.

Attrition

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

Dark Ritual

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.