msem-instigator

When Somnambulance enters, each player conjures a card named Fatigue and puts it on top of their library. (It's a colorless sorcery that costs {2} with “Draw a card.”)
Whenever a player casts a spell named Fatigue, you may draw a card. If you do, you lose 1 life.

Target player discards two cards.

Flying, lifelink
Morph {4}{B}
When Soul Cajoler is turned face up, manifest the bottom card of your library. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)

Target creature gets -3/-3 until end of turn. If you control a legendary creature, instead destroy that creature.

Menace
Inspired — Whenever Soul Engulfer becomes untapped, each opponent loses 4 life.

Destroy target creature unless its controller pays {3}.

Target opponent loses 3 life and you gain 3 life. Scry 1.

Target creature gets -3/-3 until end of turn. You gain 3 life.

{1}, Sacrifice a creature: Each opponent loses 1 life and you gain 1 life.

Poisonous 3 (Whenever this creature deals combat damage to a player, that player gets three poison counters.)
When Soulbreaker of Mautakesh enters, target poisoned player sacrifices a creature.

Enspirit (As you cast this spell, you may exile a nonenchantment creature you control until this leaves the battlefield or fails to resolve. Reduce this spell's cost by the exiled card's mana value.)
Haste

Destroy target creature. Then if you control a Wizard, that creature's controller loses 1 life for each creature card in their graveyard.

Mimic—Discard a card (Discard a card as a sorcery: This takes the shape of your choice.) [1/1]
[Shape 1] Menace
Soulshifter gets +1/+1 for each creature card in your graveyard. [1/1]
[Shape 2] Deathtouch
Whenever Soulshifter deals combat damage to a player, you may return target creature card from your graveyard to your hand. [2/2]

Sacrifice a creature: Soulsorrow Shade gets +2/+2 until end of turn.

Spend only black mana on X.
Any number of target players each draw X cards and lose X life.

All creatures get -2/-2 until end of turn. At the beginning of the next end step, if three or more creatures died this turn, create two tapped Treasure tokens.

Menace
When Spaceship Invader enters, destroy target creature you don't control with mana value 3 or less.

Whenever you activate one of Ai's loyalty abilities, create a 2/2 colorless Soldier artifact creature token with that ability, except it costs +1 to activate.
[+1]: Ai deals 1 damage to target player or planeswalker. You gain 1 life.
[–3]: Draw a card. You lose 1 life.
[–7]: Each opponent sacrifices a permanent and discards a card.

Flying, trample
Whenever an opponent casts a noncreature spell, that player loses 5 life unless they sacrifice a creature or discard a card.

Menace (This creature can't be blocked except by two or more creatures.)
When Spawn of Shadows enters, each player loses 4 life.

Manifest all cards from all players' hands. (To manifest a card, put it onto the battlefield face down as a 2/2 creature. Turn it face up any time for its mana cost if it's a creature card.)

Menace
Whenever you cast a cryptic spell, you may return target creature card from your graveyard to your hand. (Artifacts, enchantments and modal spells are cryptic.)

Golden age (If you cast an instant or sorcery spell, activated a nonmana ability, and attacked this turn, enter a renaissance for this game.)
Target creature gets -2/-2 until end of turn. If you're in a renaissance, it gets -5/-5 until end of turn instead.

When Specimen Collector enters, you may salvage a creature card. (Exile one from your graveyard until this creature leaves the battlefield. Whenever this creature deals combat damage to a player, put a card it salvaged into your hand.)

Rouse 4. Sacrifice a creature at the beginning of the next end step. (Distribute four +1/+1 counters among permanents you control. If they aren't creatures, tap them and they become 0/0 creatures in addition to their other types.)