Impact Tremors

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

Warstorm Surge

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

Path of Discovery

Whenever a creature you control enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on the creature, then put the card back or put it into your graveyard.)

Leyline of Vitality

If this card is in your opening hand, you may begin the game with it on the battlefield.
Creatures you control get +0/+1.
Whenever a creature you control enters, you may gain 1 life.

In the Web of War

Whenever a creature you control enters, it gets +2/+0 and gains haste until end of turn.

Xyris, the Writhing Storm

Flying
Whenever an opponent draws a card except the first one they draw in each of their draw steps, create a 1/1 green Snake creature token.
Whenever Xyris deals combat damage to a player, you and that player each draw that many cards.

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.