Cabal Ritual

Add {B}{B}{B}.
Threshold — Add {B}{B}{B}{B}{B} instead if seven or more cards are in your graveyard.

Gixian Skullflayer

At the beginning of your upkeep, if there are three or more creature cards in your graveyard, put a +1/+1 counter on Gixian Skullflayer.

Carrion Feeder

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

Dark Ritual

Gurmag Angler

Delve (Each card you exile from your graveyard while casting this spell pays for {1}.)

Gorging Vulture

Flying
When Gorging Vulture enters the battlefield, mill four cards. You gain 1 life for each creature card milled this way. (To mill four cards, put the top four cards of your library into your graveyard.)

Viscera Seer

Sacrifice a creature: Scry 1. (Look at the top card of your library. You may put that card on the bottom of your library.)

Rank Officer

When Rank Officer enters the battlefield, you may discard a card. If you do, create a 2/2 black Zombie creature token.
{1}{B}, {T}, Exile a creature card from your graveyard: Each opponent loses 2 life.

Balustrade Spy

Flying
When Balustrade Spy enters the battlefield, target player reveals cards from the top of their library until they reveal a land card, then puts those cards into their graveyard.

Swamp

First-Sphere Gargantua

When First-Sphere Gargantua enters the battlefield, you draw a card and you lose 1 life.
Unearth {2}{B} ({2}{B}: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the next end step or if it would leave the battlefield. Unearth only as a sorcery.)

Overwhelming Remorse

This spell costs {1} less to cast for each creature card in your graveyard.
Exile target creature or planeswalker.

Writhing Necromass

This spell costs {1} less to cast for each creature card in your graveyard.
Deathtouch

Songs of the Damned

Add {B} for each creature card in your graveyard.

Deadly Dispute

As an additional cost to cast this spell, sacrifice an artifact or creature.
Draw two cards and create a Treasure token. (It's an artifact with "{T}, Sacrifice this artifact: Add one mana of any color.")

Stinkweed Imp

Flying
Whenever Stinkweed Imp deals combat damage to a creature, destroy that creature.
Dredge 5 (If you would draw a card, you may mill five cards instead. If you do, return this card from your graveyard to your hand.)

Thorn of the Black Rose

Deathtouch
When Thorn of the Black Rose enters the battlefield, you become the monarch.