msem-instigator

Search target opponent's library for a card with mana value 3 or less and exile it with a smoke counter on it. Then that player shuffles. You may play that card for as long as it remains exiled, and you may spend mana as if it were any color to cast it.

Equipped creature gets +0/+2 and has menace.
Equip {1}

Enchant creature
Enchanted creature has poisonous 3. (Whenever that creature deals combat damage to a player, that player gets three poison counters.)

Flash
Wither (This deals damage to creatures in the form of -1/-1 counters.)

Scry 2 if a player lost life this turn. You draw two cards and you lose 2 life.

Target opponent discards two cards. You gain 2 life. (Then exile this card. You may cast the creature later from exile.)

Target creature gets -X/-X until end of turn, where X is the number of cards in your graveyard. If that creature would die this turn, exile it instead.
Cycling {B} ({B}, Discard this card: Draw a card.)

Target opponent separates all creatures into two piles. You choose one of those piles. For each creature in the chosen pile, its controller sacrifices it.

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.