Slaughter Specialist

When Slaughter Specialist enters, each opponent creates a 1/1 white Human creature token.
Whenever a creature an opponent controls dies, put a +1/+1 counter on Slaughter Specialist.

Vector Asp

{B}: Vector Asp gains infect until end of turn. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)

Yargle, Glutton of Urborg

Nightshade Harvester

Whenever a land an opponent controls enters, that player loses 1 life. Put a +1/+1 counter on Nightshade Harvester.

Mortician Beetle

Whenever a player sacrifices a creature, you may put a +1/+1 counter on Mortician Beetle.

Darksteel Myr

Indestructible (Damage and effects that say "destroy" don't destroy this creature. If its toughness is 0 or less, it's still put into its owner's graveyard.)

Kefnet the Mindful

Flying, indestructible
Kefnet the Mindful can't attack or block unless you have seven or more cards in hand.
{3}{U}: Draw a card, then you may return a land you control to its owner's hand.

Butcher Ghoul

Undying (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.)

Invisible Stalker

Hexproof (This creature can't be the target of spells or abilities your opponents control.)
Invisible Stalker can't be blocked.

Carrion Feeder

Carrion Feeder can't block.
Sacrifice a creature: Put a +1/+1 counter on Carrion Feeder.

Cabal Initiate

Discard a card: Cabal Initiate gains lifelink until end of turn.
Threshold — Cabal Initiate gets +1/+2 as long as seven or more cards are in your graveyard.

Rotting Regisaur

At the beginning of your upkeep, discard a card.

Leveler

When Leveler enters, exile all cards from your library.

Cordial Vampire

Whenever Cordial Vampire or another creature dies, put a +1/+1 counter on each Vampire you control.

Yahenni, Undying Partisan

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

Eater of Days

Flying, trample
When Eater of Days enters, you skip your next two turns.

Nyxathid

As Nyxathid enters, choose an opponent.
Nyxathid gets -1/-1 for each card in the chosen player's hand.

Lazav, the Multifarious

When Lazav, the Multifarious enters, surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
{X}: Lazav, the Multifarious becomes a copy of target creature card in your graveyard with mana value X, except its name is Lazav, the Multifarious, it's legendary in addition to its other types, and it has this ability.