Hapatra, Vizier of Poisons

Whenever Hapatra deals combat damage to a player, you may put a -1/-1 counter on target creature.
Whenever you put one or more -1/-1 counters on a creature, create a 1/1 green Snake creature token with deathtouch.

Carnifex Demon

Flying
This creature enters with two -1/-1 counters on it.
{B}, Remove a -1/-1 counter from this creature: Put a -1/-1 counter on each other creature.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Kulrath Knight

Flying
Wither (This deals damage to creatures in the form of -1/-1 counters.)
Creatures your opponents control with counters on them can't attack or block.

Blowfly Infestation

Whenever a creature dies, if it had a -1/-1 counter on it, put a -1/-1 counter on target creature.

Auntie Ool, Cursewretch

Ward—Blight 2. (To blight 2, a player puts two -1/-1 counters on a creature they control.)
Whenever one or more -1/-1 counters are put on a creature, draw a card if you control that creature. If you don't control it, its controller loses 1 life.

Devoted Druid

{T}: Add {G}.
Put a -1/-1 counter on this creature: Untap this creature.

Sinister Gnarlbark

At the beginning of your end step, draw a card and blight 1. (Put a -1/-1 counter on a creature you control.)

Command Tower

The Reaper, King No More

When The Reaper enters, put a -1/-1 counter on each of up to two target creatures.
Whenever a creature an opponent controls with a -1/-1 counter on it dies, you may put that card onto the battlefield under your control. Do this only once each turn.

Dread Tiller

When this creature enters, put a -1/-1 counter on target creature.
Whenever a creature with a -1/-1 counter on it dies, you may put a land card from your hand or graveyard onto the battlefield tapped.

Farseek

Search your library for a Plains, Island, Swamp, or Mountain card, put it onto the battlefield tapped, then shuffle.

Necroskitter

Wither (This deals damage to creatures in the form of -1/-1 counters.)
Whenever a creature an opponent controls with a -1/-1 counter on it dies, you may return that card to the battlefield under your control.

The Scorpion God

Whenever a creature with a -1/-1 counter on it dies, draw a card.
{1}{B}{R}: Put a -1/-1 counter on another target creature.
When The Scorpion God dies, return it to its owner's hand at the beginning of the next end step.

Soul Snuffers

When this creature enters, put a -1/-1 counter on each creature.

Village Pillagers

Wither (This deals damage to creatures in the form of -1/-1 counters.)
When this creature enters, it deals 1 damage to each creature your opponents control.
Whenever a creature an opponent controls with a counter on it dies, you create a tapped Treasure token.

Massacre Girl, Known Killer

Menace
Creatures you control have wither. (They deal damage to creatures in the form of -1/-1 counters.)
Whenever a creature an opponent controls dies, if its toughness was less than 1, draw a card.