Festerleech

Whenever Festerleech deals combat damage to a player, you mill two cards.
{1}{B}: Festerleech gets +2/+2 until end of turn. Activate only once each turn.

Durable Coilbug

{4}{B}: Return Durable Coilbug from your graveyard to your hand.

Carrion Beetles

{2}{B}, {T}: Exile up to three target cards from a single graveyard.

Devouring Swarm

Flying
Sacrifice a creature: Devouring Swarm gets +1/+1 until end of turn.

Mind Maggots

When Mind Maggots enters, discard any number of creature cards. For each card discarded this way, put two +1/+1 counters on Mind Maggots.

Bilious Skulldweller

Deathtouch
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)

Huskburster Swarm

This spell costs {1} less to cast for each creature card you own in exile and in your graveyard.
Menace, deathtouch

Blessing of Leeches

Flash
Enchant creature
At the beginning of your upkeep, you lose 1 life.
{0}: Regenerate enchanted creature.

Insidious Bookworms

When Insidious Bookworms dies, you may pay {1}{B}. If you do, target player discards a card at random.

Blightbeetle

Protection from green (This creature can't be blocked, targeted, dealt damage, enchanted, or equipped by anything green.)
Creatures your opponents control can't have +1/+1 counters put on them.

Ferropede

Ferropede can't be blocked.
Whenever Ferropede deals combat damage to a player, you may remove a counter from target permanent.

Leeching Lurker

Lifelink
Nightbound (If a player casts at least two spells during their own turn, it becomes day next turn.)

Curse of Leeches

Enchant player
As this permanent transforms into Curse of Leeches, attach it to a player.
At the beginning of enchanted player's upkeep, they lose 1 life and you gain 1 life.
Daybound (If a player casts no spells during their own turn, it becomes night next turn.)

Curse of Leeches // Leeching Lurker

Endless Cockroaches

When Endless Cockroaches dies, return it to its owner's hand.

Abundant Maw

Emerge {6}{B} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When you cast this spell, target opponent loses 3 life and you gain 3 life.

Gluttonous Slug

Menace
Evolve (Whenever a creature you control enters, if that creature has greater power or toughness than this creature, put a +1/+1 counter on this creature.)

Squelching Leeches

Squelching Leeches's power and toughness are each equal to the number of Swamps you control.

Mana Skimmer

Flying
Whenever Mana Skimmer deals damage to a player, tap target land that player controls. That land doesn't untap during its controller's next untap step.

Distended Mindbender

Emerge {5}{B}{B} (You may cast this spell by sacrificing a creature and paying the emerge cost reduced by that creature's mana value.)
When you cast this spell, target opponent reveals their hand. You choose from it a nonland card with mana value 3 or less and a card with mana value 4 or greater. That player discards those cards.

Brain Weevil

Intimidate (This creature can't be blocked except by artifact creatures and/or creatures that share a color with it.)
Sacrifice Brain Weevil: Target player discards two cards. Activate only as a sorcery.

Skullslither Worm

When Skullslither Worm enters, each opponent discards a card. For each opponent who can't, put two +1/+1 counters on Skullslither Worm.

Fumulus, the Infestation

Flying, deathtouch
Whenever a player sacrifices a nontoken creature, create a 1/1 black Insect creature token with flying.
Whenever an Insect, Leech, Slug, or Worm you control attacks, defending player loses 1 life and you gain 1 life.

Dross Hopper

Sacrifice a creature: Dross Hopper gains flying until end of turn.

Flow of Maggots

Cumulative upkeep {1} (At the beginning of your upkeep, put an age counter on this permanent, then sacrifice it unless you pay its upkeep cost for each age counter on it.)
Flow of Maggots can't be blocked by non-Wall creatures.

Brain Maggot

When Brain Maggot enters, target opponent reveals their hand and you choose a nonland card from it. Exile that card until Brain Maggot leaves the battlefield.

Brood of Cockroaches

When Brood of Cockroaches is put into your graveyard from the battlefield, at the beginning of the next end step, you lose 1 life and return Brood of Cockroaches to your hand.

Leech Gauntlet

Lifelink
Equipped creature has lifelink.
Reconfigure {4} ({4}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Fog of Gnats

Deadly Grub

Vanishing 3 (This creature enters with three time counters on it. At the beginning of your upkeep, remove a time counter from it. When the last is removed, sacrifice it.)
When Deadly Grub dies, if it had no time counters on it, create a 6/1 green Insect creature token with shroud. (It can't be the target of spells or abilities.)

Bane of the Living

Morph {X}{B}{B} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up any time for its morph cost.)
When Bane of the Living is turned face up, all creatures get -X/-X until end of turn.

Leechridden Swamp

({T}: Add {B}.)
Leechridden Swamp enters tapped.
{B}, {T}: Each opponent loses 1 life. Activate only if you control two or more black permanents.

Carrion Grub

Carrion Grub gets +X/+0, where X is the greatest power among creature cards in your graveyard.
When Carrion Grub enters, mill four cards. (Put the top four cards of your library into your graveyard.)

Battlefly Swarm

Flying
{B}: Battlefly Swarm gains deathtouch until end of turn.

Fretwork Colony

Fretwork Colony can't block.
At the beginning of your upkeep, put a +1/+1 counter on Fretwork Colony and you lose 1 life.

Mana Leech

You may choose not to untap Mana Leech during your untap step.
{T}: Tap target land. It doesn't untap during its controller's untap step for as long as Mana Leech remains tapped.

Caustic Crawler

Landfall — Whenever a land you control enters, you may have target creature get -1/-1 until end of turn.

Slimy Dualleech

At the beginning of combat on your turn, target creature you control with power 2 or less gets +1/+0 and gains deathtouch until end of turn.