Norin the Wary

When a player casts a spell or a creature attacks, exile Norin the Wary. Return it to the battlefield under its owner's control at the beginning of the next end step.

Impact Tremors

Whenever a creature you control enters, this enchantment deals 1 damage to each opponent.

Witty Roastmaster

Alliance — Whenever another creature you control enters, Witty Roastmaster deals 1 damage to each opponent.

Rocco, Cabaretti Caterer

When Rocco, Cabaretti Caterer enters, if you cast it, you may search your library for a creature card with mana value X or less, put it onto the battlefield, then shuffle.

Purphoros, God of the Forge

Indestructible
As long as your devotion to red is less than five, Purphoros isn't a creature.
Whenever another creature you control enters, Purphoros deals 2 damage to each opponent.
{2}{R}: Creatures you control get +1/+0 until end of turn.