msem-instigator

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.

Until end of turn, target creature you control gets +1/+1 and target creature an opponent controls gets -1/-1.

Skulking Claw can't be blocked by creatures with greater power.
Skulking Claw gets +1/+0 for each unblocked attacking creature you control.

Creatures with power less than Skull-Crowned Chief's power can't block.

Flying
Death knell — Skullhoarder Dragon gets +2/+0 until end of turn. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Whenever a creature dealt damage by Slave Child this turn dies, you may pay {B}. If you do, transform Slave Child.

This spell costs {1}{B} less to cast if it targets a creature with indestructible.
Exile target creature.

Whenever a creature attacks you or a planeswalker you control, that creature's controller mills two cards.

{T}: Target opponent loses 2 life. Activate only if you gained life this turn.

Surveil 2. When you do, destroy up to one target creature with power less than or equal to the number of card types among cards in your graveyard.

Advance {5}{B}{B} (Cast this for {5}{B}{B} to advance it as it enters. {5}{B}{B}: Advance this as a sorcery.)
Whenever Sluiceway Collector advances, put a permanent card from a graveyard onto the battlefield under your control.

Distorted (This creature can be blocked by tapped creatures but not untapped creatures.)
{2}{B}, Sacrifice Sluiceway Shrieker: Put the bottom card of your library into your hand.

Deathtouch
When Slum Scrounger enters, put up to two target creature cards in your graveyard on top of your library in any order.

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Slums Thug gets +1/+1 and has menace as long as you've gained life this turn.

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.

You draw a card and you lose 1 life. Exile Small Luxuries.
At the beginning of your upkeep, you may return Small Luxuries from your graveyard to your hand.

When Smirk enters, add the top two cards of your library to your evil plan. (Look at them, then exile them face-down. You may pay {7} once a turn as a sorcery to cast any number of cards in your evil plan without paying their mana costs.)
Whenever you cast your second spell each turn, you draw a card and you lose 2 life.

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.