Brainstorm

Draw three cards, then put two cards from your hand on top of your library in any order.

Tekuthal, Inquiry Dominus

Flying
If you would proliferate, proliferate twice instead.
{1}{U/P}{U/P}, Remove three counters from among other artifacts, creatures, and planeswalkers you control: Put an indestructible counter on Tekuthal, Inquiry Dominus. ({U/P} can be paid with either {U} or 2 life.)

Counterspell

Bloated Contaminator

Trample
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
Whenever Bloated Contaminator deals combat damage to a player, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Bring the Ending

Counter target spell unless its controller pays {2}.
Corrupted — Counter that spell instead if its controller has three or more poison counters.

Flux Channeler

Whenever you cast a noncreature spell, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Phyresis Outbreak

Each opponent gets a poison counter. Then each creature your opponents control gets -1/-1 until end of turn for each poison counter its controller has.

Thrummingbird

Flying
Whenever Thrummingbird deals combat damage to a player, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Contaminant Grafter

Trample, toxic 1
Whenever one or more creatures you control deal combat damage to one or more players, proliferate.
Corrupted — At the beginning of your end step, if an opponent has three or more poison counters, draw a card, then you may put a land card from your hand onto the battlefield.

Cankerbloom

{1}, Sacrifice Cankerbloom: Choose one —
• Destroy target artifact.
• Destroy target enchantment.
• Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Fynn, the Fangbearer

Deathtouch (Any amount of damage this deals to a creature is enough to destroy it.)
Whenever a creature you control with deathtouch deals combat damage to a player, that player gets two poison counters. (A player with ten or more poison counters loses the game.)

Venerated Rotpriest

Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
Whenever a creature you control becomes the target of a spell, target opponent gets a poison counter.

Whisper of the Dross

Target creature gets -1/-1 until end of turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Vraska's Fall

Each opponent sacrifices a creature or planeswalker of their choice and gets a poison counter.

Metastatic Evangel

Whenever another nontoken creature you control enters, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Blighted Agent

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Blighted Agent can't be blocked.

Plague Stinger

Flying
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Infectious Bite

Target creature you control deals damage equal to its power to target creature you don't control. Each opponent gets a poison counter.

Atraxa, Praetors' Voice

Flying, vigilance, deathtouch, lifelink
At the beginning of your end step, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Persistent Constrictor

At the beginning of each opponent's upkeep, they lose 1 life and you put a -1/-1 counter on up to one target creature they control.
Persist (When this creature dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Grateful Apparition

Flying
Whenever Grateful Apparition deals combat damage to a player or planeswalker, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Evolution Sage

Landfall — Whenever a land you control enters, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Prologue to Phyresis

Each opponent gets a poison counter.
Draw a card.

Experimental Augury

Look at the top three cards of your library. Put one of them into your hand and the rest on the bottom of your library in any order. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Viridian Corrupter

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
When Viridian Corrupter enters, destroy target artifact.

Dreamtide Whale

Vanishing 2 (This creature enters with two time counters on it. At the beginning of your upkeep, remove a time counter from it. When the last is removed, sacrifice it.)
Whenever a player casts their second spell each turn, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Bilious Skulldweller

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

Infectious Inquiry

You draw two cards and you lose 2 life. Each opponent gets a poison counter.

Corrupted Resolve

Counter target spell if its controller is poisoned.

Ichor Rats

Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
When Ichor Rats enters, each player gets a poison counter.

Slaughter Singer

Toxic 2 (Players dealt combat damage by this creature also get two poison counters.)
Whenever another creature you control with toxic attacks, it gets +1/+1 until end of turn.

Bloodroot Apothecary

Toxic 2 (Players dealt combat damage by this creature also get two poison counters. A player with ten or more poison counters loses the game.)
When Bloodroot Apothecary enters, you and target opponent each create a Treasure token.
Whenever an opponent sacrifices a noncreature token, that player gets two poison counters.

Glistening Sphere

Glistening Sphere enters tapped.
When Glistening Sphere enters, proliferate.
{T}: Add one mana of any color.
Corrupted — {T}: Add three mana of any one color. Activate only if an opponent has three or more poison counters.

Reject Imperfection

Counter target spell. If that spell's mana value was 3 or less, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)