Sludge Monster

Whenever this creature enters or attacks, put a slime counter on up to one other target creature.
Non-Horror creatures with slime counters on them lose all abilities and have base power and toughness 2/2.

Fleeting Reflection

Target creature you control gains hexproof until end of turn. Untap that creature. Until end of turn, it becomes a copy of up to one other target creature.

Kasmina's Transmutation

Enchant creature
Enchanted creature loses all abilities and has base power and toughness 1/1.

Dramatic Accusation

Enchant creature
When this Aura enters, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.
{U}{U}: Shuffle enchanted creature into its owner's library.

Bloodline Culling

Choose one —
• Target creature gets -5/-5 until end of turn.
• Creature tokens get -2/-2 until end of turn.

Massacre Girl

Menace
When Massacre Girl enters, each other creature gets -1/-1 until end of turn. Whenever a creature dies this turn, each creature other than Massacre Girl gets -1/-1 until end of turn.

Mind Flayer

Dominate Monster — When this creature enters, gain control of target creature for as long as you control this creature.

Zulaport Duelist

Flash
When this creature enters, up to one target creature gets -2/-0 until end of turn. Its controller mills two cards. (They put the top two cards of their library into their graveyard.)

Pin to the Earth

Enchant creature
Enchanted creature gets -6/-0.

Cleric of Chill Depths

Whenever this creature blocks a creature, that creature doesn't untap during its controller's next untap step.

Swamp

Island

Toxrill, the Corrosive

At the beginning of each end step, put a slime counter on each creature you don't control.
Creatures you don't control get -1/-1 for each slime counter on them.
Whenever a creature you don't control with a slime counter on it dies, create a 1/1 black Slug creature token.
{U}{B}, Sacrifice a Slug: Draw a card.

Aspect of Gorgon

Enchant creature
Enchanted creature gets +1/+3 and has deathtouch. (Any amount of damage it deals to a creature is enough to destroy it.)

Massacre Wurm

When this creature enters, creatures your opponents control get -2/-2 until end of turn.
Whenever a creature an opponent controls dies, that player loses 2 life.

Hermitic Nautilus

Vigilance
{1}{U}: This creature gets +3/-3 until end of turn.

Shimmerwing Chimera

Flying
At the beginning of your upkeep, return up to one other target enchantment you control to its owner's hand.

Blightcaster

Whenever you cast an enchantment spell, you may have target creature get -2/-2 until end of turn.

Midnight Assassin

Flying
Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)

Shambling Ghast

When this creature dies, choose one —
• Target creature an opponent controls gets -1/-1 until end of turn.
• Create a Treasure token. (It's an artifact with "{T}, Sacrifice this token: Add one mana of any color.")

Dampening Pulse

Creatures your opponents control get -1/-0.

Grim Guardian

Constellation — Whenever this creature or another enchantment you control enters, each opponent loses 1 life.

Night Clubber

When this creature enters, creatures your opponents control get -1/-1 until end of turn.
Blitz {2}{B} (If you cast this spell for its blitz cost, it gains haste and "When this creature dies, draw a card." Sacrifice it at the beginning of the next end step.)

Stab Wound

Enchant creature
Enchanted creature gets -2/-2.
At the beginning of the upkeep of enchanted creature's controller, that player loses 2 life.

Threnody Singer

Flash
Flying
When this creature enters, target creature an opponent controls gets -X/-0 until end of turn, where X is your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Clawing Torment

Enchant artifact or creature
As long as enchanted permanent is a creature, it gets -1/-1 and can't block.
Enchanted permanent has "At the beginning of your upkeep, you lose 1 life."

Slug

Gisa, Glorious Resurrector

If a creature an opponent controls would die, exile it instead.
At the beginning of your upkeep, put all creature cards exiled with Gisa onto the battlefield under your control. They gain decayed. (A creature with decayed can't block. When it attacks, sacrifice it at end of combat.)

Witness Protection

Enchant creature
Enchanted creature loses all abilities and is a green and white Citizen creature with base power and toughness 1/1 named Legitimate Businessperson. (It loses all other colors, card types, creature types, and names.)

Tightening Coils

Enchant creature
Enchanted creature gets -6/-0 and loses flying.

Grafted Identity

As an additional cost to cast this spell, sacrifice a creature.
Enchant creature
You control enchanted creature.
Enchanted creature gets +1/+1.

Treasure

{T}, Sacrifice this token: Add one mana of any color.

Agoraphobia

Enchant creature
Enchanted creature gets -5/-0.
{2}{U}: Return this Aura to its owner's hand.

Dismal Backwater

This land enters tapped.
When this land enters, you gain 1 life.
{T}: Add {U} or {B}.

Serpentine Ambush

Until end of turn, target creature becomes a blue Serpent with base power and toughness 5/5.

Grasp of Darkness

Target creature gets -4/-4 until end of turn.

So Tiny

Flash
Enchant creature
Enchanted creature gets -2/-0. It gets -6/-0 instead as long as its controller has seven or more cards in their graveyard.