msem-instigator

When Herald of Darkness enters, destroy up to one target creature with mana value 3 or less.

When Kill This Love enters, target opponent sacrifices a creature.
At the beginning of each end step, if a creature died this turn, you gain 2 life.

Return target creature card from your graveyard to your hand. If you cast Lift Weary Heads from Rest from the graveyard, put it onto the battlefield instead.
Reconstruct {6}{B}{B} (You may cast this from your graveyard for its reconstruct cost reduced by the mana value of a permanent you sacrifice as you cast it this way. Then exile this.)

Lost Explorer has first strike as long as it's attacking.
{1}{B}, Exile Lost Explorer from your graveyard: Crystallize. (Put a +1/+1 counter on a creature you control or create a Treasure.)

Search your library for a creature or sorcery card, put it into your graveyard, then shuffle.
Reconstruct {2}{B} (You may cast this from your graveyard for its reconstruct cost reduced by the mana value of a permanent you sacrifice as you cast it this way. Then exile this.)

Deathtouch
Whenever Operative SCARY attacks or you cast a green or white spell, you may draw a card. If you do, you lose 2 life.

When Outside the Airlock enters, create a tapped 3/2 green Horror creature token.
Whenever a creature you control dies, each opponent loses 1 life and you gain 1 life.

When P-3 Ambusher enters, you may sacrifice a creature or discard a creature card. If you do, draw a card.

Each player chooses a nonbasic card name, then searches each of their opponents' graveyards, hands, and libraries for any number of cards with the chosen name and exiles them. Each player shuffles, then draws a card for each card exiled from their hand this way.

Predatory Abomination has deathtouch and lifelink as long as you control another Horror.

[+1]: Remove all counters from target permanent.
[+1]: Each opponent discards a card.
[–2]: Exile target creature or planeswalker card from your graveyard with mana value less than or equal to the number of lands you control. If you do, create a token that's a copy of it.
[–7]: Each opponent gets an emblem with “At the beginning of your upkeep, you lose 7 life unless you sacrifice seven permanents.”

When Skittering Atrocity enters, up to one target creature gets -X/-X until end of turn, where X is the number of Horrors you control.

When Slyten Analyst enters, you may search your library for a card, then shuffle and put that card on top. If you do, you lose 2 life.

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

At the beginning of your end step, you may sacrifice a permanent. When you do, return target creature card from your graveyard to your hand.

Target creature gets +2/+0 until end of turn and gains “When this creature dies, return it to the battlefield tapped under its owner's control.”

As an additional cost to cast this spell, exile any number of cards from your graveyard. This spell costs {1} less to cast for each card exiled this way.
All creatures get -3/-3 until end of turn.

When Twisted Predator dies, you draw a card and you lose 1 life.

This spell costs {1} less to cast for each artifact you control. (This applies before Treasures are sacrificed for mana.)
When Underhanded Deal enters, draw three cards.
Whenever you cast a spell, you lose 1 life.

Rhapsody {4}{R}{R} (Pay {4}{R}{R} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
Trample
Whenever Attuned Slayer deals combat damage to a player, exile that many cards from the top of your library. You may play those cards this turn.

Haste
Augmented — At the beginning of combat on your turn, if you've spent {W}{U}{B}{R}{G} to cast spells this turn, gain control of target nonland permanent until end of turn. Untap it. It gains haste until end of turn.

This spell costs {3} less to cast if it targets a permanent with mana value 3 or less.
Gain control of target nonland permanent until end of turn. Untap it. It gains haste until end of turn.

Rhapsody {5}{R}{R} (Pay {5}{R}{R} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
When Corrupted Guardian enters, it deals damage equal to its power to up to one target player or planeswalker.

When Expedition Outfitter enters, create a 1/1 colorless Servo artifact creature token.
At the beginning of each end step, if an artifact was put into your graveyard from the battlefield this turn, Expedition Outfitter deals 1 damage to each opponent.

When Extraction Mercenary enters, if you attacked this turn, crystallize. (Put a +1/+1 counter on a creature you control or create a Treasure.)
{4}{R}{R}: Extraction Mercenary deals 3 damage to any target.