Iron Apprentice

This creature enters with a +1/+1 counter on it.
When this creature dies, if it had counters on it, put those counters on target creature you control.

Sign in Blood

Target player draws two cards and loses 2 life.

Marchesa, the Black Rose

Dethrone (Whenever this creature attacks the player with the most life or tied for most life, put a +1/+1 counter on it.)
Other creatures you control have dethrone.
Whenever a creature you control with a +1/+1 counter on it dies, return that card to the battlefield under your control at the beginning of the next end step.

Carrion Feeder

This creature can't block.
Sacrifice a creature: Put a +1/+1 counter on this creature.

Swamp

Island

Ashnod's Altar

Feast on the Fallen

At the beginning of each upkeep, if an opponent lost life last turn, put a +1/+1 counter on target creature you control.

Yahenni, Undying Partisan

Haste
Whenever a creature an opponent controls dies, put a +1/+1 counter on Yahenni.
Sacrifice another creature: Yahenni gains indestructible until end of turn.

Arcane Signet

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

Spawning Pit

Sacrifice a creature: Put a charge counter on this artifact.
{1}, Remove two charge counters from this artifact: Create a 2/2 colorless Spawn artifact creature token.

Metallic Mimic

As this creature enters, choose a creature type.
This creature is the chosen type in addition to its other types.
Each other creature you control of the chosen type enters with an additional +1/+1 counter on it.

Morbid Opportunist

Whenever one or more other creatures die, draw a card. This ability triggers only once each turn.

Unspeakable Symbol

Pay 3 life: Put a +1/+1 counter on target creature.

Uncivil Unrest

Nontoken creatures you control have riot. (They enter with your choice of a +1/+1 counter or haste.)
If a creature you control with a +1/+1 counter on it would deal damage to a permanent or player, it deals double that damage instead.

Canonized in Blood

At the beginning of your end step, if you descended this turn, put a +1/+1 counter on target creature you control. (You descended if a permanent card was put into your graveyard from anywhere.)
{5}{B}{B}, Sacrifice this enchantment: Create a 4/3 white and black Vampire Demon creature token with flying.

Kalain, Reclusive Painter

When Kalain enters, create a Treasure token. (It's an artifact with "{T}, Sacrifice this token: Add one mana of any color.")
Other creatures you control enter with an additional +1/+1 counter on them for each mana from a Treasure spent to cast them.

Mountain

Fellwar Stone

{T}: Add one mana of any color that a land an opponent controls could produce.

Spinal Centipede

When this creature dies, put a +1/+1 counter on target creature you control.

Viscera Seer

Sacrifice a creature: Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Thran Vigil

Whenever one or more artifact and/or creature cards leave your graveyard during your turn, put a +1/+1 counter on target creature you control.

Flesh-Eater Imp

Flying
Infect (This creature deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Sacrifice a creature: This creature gets +1/+1 until end of turn.

Sol Ring

Bloodtracker

Flying
{B}, Pay 2 life: Put a +1/+1 counter on this creature.
When this creature leaves the battlefield, draw a card for each +1/+1 counter on it.