Pandemonium

Whenever a creature enters, that creature's controller may have it deal damage equal to its power to any target of their choice.

Darien, King of Kjeldor

Whenever you're dealt damage, you may create that many 1/1 white Soldier creature tokens.

Whenever a creature enters, you may add an amount of {C} equal to the number of creatures you control that share a creature type with it.

{T}: Heartless Hidetsugu deals damage to each player equal to half that player's life total, rounded down.

{T}, Sacrifice a creature: This artifact deals 1 damage to any target.
Whenever a creature enters, you may untap this artifact.

Whenever a creature you control enters, it deals damage equal to its power to any target.

Whenever a player casts a spell with mana value 3 or less, this creature deals 2 damage to that player.

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