Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Voldaren Epicure

When Voldaren Epicure enters, it deals 1 damage to each opponent. Create a Blood token. (It's an artifact with "{1}, {T}, Discard a card, Sacrifice this artifact: Draw a card.")

Lava Zombie

When Lava Zombie enters, return a black or red creature you control to its owner's hand.
{2}: Lava Zombie gets +1/+0 until end of turn.

Blister Beetle

When Blister Beetle enters, target creature gets -1/-1 until end of turn.

Ardent Elementalist

When Ardent Elementalist enters, return target instant or sorcery card from your graveyard to your hand.

Chittering Rats

When Chittering Rats enters, target opponent puts a card from their hand on top of their library.