Kaervek the Merciless

Whenever an opponent casts a spell, Kaervek the Merciless deals damage to any target equal to that spell’s converted mana cost.

Darkheart Sliver

All Slivers have “Sacrifice this permanent: You gain 3 life.”

Ridged Kusite

, , Discard a card: Target creature gets +1/+0 and gains first strike until end of turn.

Shaper Parasite

Morph (You may cast this card face down as a 2/2 creature for . Turn it face up any time for its morph cost.)
When Shaper Parasite is turned face up, target creature gets +2/-2 or -2/+2 until end of turn.

Tidehollow Sculler

When Tidehollow Sculler enters the battlefield, target opponent reveals their hand and you choose a nonland card from it. Exile that card.
When Tidehollow Sculler leaves the battlefield, return the exiled card to its owner’s hand.

Simic Sky Swallower

Flying, trample
Shroud (This creature can’t be the target of spells or abilities.)

Batterskull

Living weapon (When this Equipment enters the battlefield, create a 0/0 black Germ creature token, then attach this to it.)
Equipped creature gets +4/+4 and has vigilance and lifelink.
: Return Batterskull to its owner’s hand.
Equip

Fatespinner

At the beginning of each opponent’s upkeep, that player chooses draw step, main phase, or combat phase. The player skips each instance of the chosen step or phase this turn.

Mind Funeral

Target opponent reveals cards from the top of their library until four land cards are revealed. That player puts all cards revealed this way into their graveyard.

Flame Jab

Flame Jab deals 1 damage to any target.
Retrace (You may cast this card from your graveyard by discarding a land card in addition to paying its other costs.)

Shaper Parasite

Morph (You may cast this card face down as a 2/2 creature for . Turn it face up any time for its morph cost.)
When Shaper Parasite is turned face up, target creature gets +2/-2 or -2/+2 until end of turn.

Sakashima the Impostor

You may have Sakashima the Impostor enter the battlefield as a copy of any creature on the battlefield, except its name is Sakashima the Impostor, it’s legendary in addition to its other types, and it has “: Return this creature to its owner’s hand at the beginning of the next end step.”

Overseer of the Damned

Flying
When Overseer of the Damned enters the battlefield, you may destroy target creature.
Whenever a nontoken creature an opponent controls dies, create a tapped 2/2 black Zombie creature token.

Avatar of Discord

( can be paid with either or .)
Flying
When Avatar of Discord enters the battlefield, sacrifice it unless you discard two cards.