msem-instigator

Children of the Void has hexproof as long as it's your turn.
Children of the Void can't attack or block unless you reappropriate (exile exactly two cards from your hand and/or graveyard).

This spell costs {1} less to cast for each Desert you control.
When Crested Crustacean enters the battlefield, return target permanent to its owner's hand.

Deep-Sea Terror can't attack unless seven or more cards are in your graveyard.

Whenever a creature an opponent controls deals damage to you, monstrosity 2. (If this creature isn't monstrous, put two +1/+1 counters on it and it becomes monstrous.)
Frigid Behemoth can't attack unless it's monstrous.

Defender
Civilized — As long as you control an artifact, a creature, and an enchantment, Hauling Hippopotamus can attack as though it didn't have defender.

Icecap Tortoise enters the battlefield with three ice counters on it.
Icecap Tortoise has defender as long as it has an ice counter on it.
{1}{U}: Remove an ice counter from Icecap Tortoise.

Library Specter can't attack unless there are five or more noncreature, nonland cards in your graveyard.

Flash
Oscanthor, Trader's Ruin can't be blocked.
Whenever a creature you control deals combat damage to a player, draw a card.

Return target permanent to its owner's hand.
Coalesce {7}{U} (You may cast this from your graveyard for {7}{U}. If you do, put it onto the battlefield as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

This spell can't be countered.
Flying, protection from cryptic
Whenever you cast a cryptic spell, cast a Sphinx spell, or draw a card, investigate.
{T}, Sacrifice ten Clues: You win the game.

This spell costs {1} less to cast for each enchantment you control.
Flying, lifelink, trample
Disturb {6}{B}{B}. That spell costs {1} less to cast for each enchantment you control. (You may cast this card from your graveyard transformed for its disturb cost.)

Indestructible
You may play cards from your graveyard.
If you would draw one or more cards, mill that many cards instead.
If a card would be put into your graveyard from anywhere other than your library, exile it instead.

Inspired — Whenever Bearer of Vile Incense becomes untapped, create a 0/1 black Cultist creature token with “{1}{B}{B}, Sacrifice three Cultists: Create a 5/5 black Demon creature token.”

Flying
Sacrifice six Swamps: Return Berbarus Unsealed from your graveyard to the battlefield with an indestructible counter on it. You draw six cards and you lose 6 life.

Bloodprice {4}{B} (You may cast this card by sacrificing a creature and paying its bloodprice.)
At the beginning of your upkeep, if there are five or more creature cards in your graveyard, create a 2/2 black Demon creature token.

This spell costs {2} less to cast for each creature that died this turn.
Flying, trample

{1}{B}, Sacrifice another creature: Draw a card and create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)
{2}{B}, Sacrifice three Idols: Return Demon of Hubris from your graveyard to the battlefield. Activate only as a sorcery.

When Doomstep Giant enters the battlefield, each player sacrifices a land.

This spell costs {1} less to cast for each card put into your graveyard from anywhere this turn.
At the beginning of each player's upkeep, that player loses 1 life, discards a card, and sacrifices a creature.

Flying, lifelink
Fallen Celestial can't block or be blocked by Angels, Demons, or Faeries.
At the beginning of each opponent's upkeep, that player loses 1 life and creates a 1/1 black Faerie creature token with flying.

Menace
Whenever Flayed Dunerazer attacks or blocks, sacrifice a permanent.

Bloodprice {B}{B}{B}{B} (You may cast this card by sacrificing a creature and paying its bloodprice.)
Lifelink, haste
Sacrifice another Demon: Jejun, Dark Hunter gains flying and indestructible until end of turn.

Flying, trample
At the beginning of combat on your turn, an opponent may sacrifice a creature. If they do, you may sacrifice a creature. If you do, repeat this process. If you don't, tap Kai'Jun.

Menace
When Lord of Decay enters the battlefield, put a -1/-1 counter on it and each creature you don't control.

Flying, trample
Nether Fiend enters the battlefield with a lifelink counter on it.
{B}{B}, Pay 6 life, Remove a lifelink counter from Nether Fiend: Draw six cards.