Have

Voidwing Hybrid

Flying
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
When you proliferate, return this card from your graveyard to your hand.

Serum Snare

Return target nonland permanent to its owner's hand. If that permanent had mana value 3 or less, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Jwar Isle Refuge

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

Infectious Inquiry

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

Blightbelly Rat

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

Maison des Jeunes

Swamp

Dismal Backwater

Distorted Curiosity

Corrupted — This spell costs {2} less to cast if an opponent has three or more poison counters.
Draw two cards.

Vraska's Fall

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

Dimir Guildgate

This land enters tapped.
{T}: Add {U} or {B}.

Scheming Aspirant

Whenever you proliferate, each opponent loses 2 life and you gain 2 life.

Waterfront District

This land enters tapped.
{T}: Add {U} or {B}.
{2}{U}{B}, {T}, Sacrifice this land: Draw a card.

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.

Prologue to Phyresis

Each opponent gets a poison counter.
Draw a card.

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.)

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.)

Duress

Target opponent reveals their hand. You choose a noncreature, nonland card from it. That player discards that card.

Drown in Ichor

Target creature gets -4/-4 until end of turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Anoint with Affliction

Exile target creature if it has mana value 3 or less.
Corrupted — Exile that creature instead if its controller has three or more poison counters.

Island

Negate

Thrummingbird

Flying
Whenever this creature 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.)