Have

Blightbelly Rat

Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)
When Blightbelly Rat dies, proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Don't Have

Blue Elemental Blast

Choose one —
• Counter target red spell.
• Destroy target red permanent.

Counterspell

Island

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.)

Swamp

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.)

Infectious Inquiry

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

Cast Down

Prologue to Phyresis

Each opponent gets a poison counter.
Draw a card.

Pestilent Syphoner

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

Getting

Dismal Backwater

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

Contentious Plan

Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Draw a card.

Brainstorm

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

Duress

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

Bojuka Bog

Bojuka Bog enters tapped.
When Bojuka Bog enters, exile target player's graveyard.
{T}: Add {B}.

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.

Golden Urn

At the beginning of your upkeep, you may put a charge counter on Golden Urn.
{T}, Sacrifice Golden Urn: You gain life equal to the number of charge counters on Golden Urn.

Vraska's Fall

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

Hydroblast

Choose one —
• Counter target spell if it's red.
• Destroy target permanent if it's red.