Blood Curdle

Destroy target creature. Put a menace counter on a creature you control. (It can't be blocked except by two or more creatures.)

Swamp

Unexpected Fangs

Put a +1/+1 counter and a lifelink counter on target creature.

Chittering Harvester

Mutate {4}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Whenever this creature mutates, each opponent sacrifices a creature of their choice.

Durable Coilbug

{4}{B}: Return this card from your graveyard to your hand.

Serrated Scorpion

When this creature dies, it deals 2 damage to each opponent and you gain 2 life.

Gloom Pangolin

Mysterious Egg

Whenever this creature mutates, put a +1/+1 counter on it.

Unbreakable Bond

Return target creature card from your graveyard to the battlefield with a lifelink counter on it.

Grimdancer

This creature enters with your choice of two different counters on it from among menace, deathtouch, and lifelink.

Dead Weight

Enchant creature
Enchanted creature gets -2/-2.

Cavern Whisperer

Mutate {3}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Menace (This creature can't be blocked except by two or more creatures.)
Whenever this creature mutates, each opponent discards a card.

Zagoth Mamba

Whenever this creature mutates, target creature an opponent controls gets -2/-2 until end of turn.

Boot Nipper

This creature enters with your choice of a deathtouch counter or a lifelink counter on it.

Corpse Churn

Mill three cards, then you may return a creature card from your graveyard to your hand. (To mill three cards, put the top three cards of your library into your graveyard.)

Insatiable Hemophage

Mutate {2}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Deathtouch
Whenever this creature mutates, each opponent loses X life and you gain X life, where X is the number of times this creature has mutated.

Dirge Bat

Mutate {4}{B}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Flying
Whenever this creature mutates, destroy target creature or planeswalker an opponent controls.