Numot, the Devastator

Flying
Whenever Numot, the Devastator deals combat damage to a player, you may pay {2}{R}. If you do, destroy up to two target lands.

Altar of the Brood

Whenever another permanent you control enters, each opponent mills a card.

Midnight Guard

Whenever another creature enters, untap Midnight Guard.

Pestermite

Flash
Flying
When Pestermite enters, you may tap or untap target permanent.

Deceiver Exarch

Flash (You may cast this spell any time you could cast an instant.)
When Deceiver Exarch enters, choose one —
• Untap target permanent you control.
• Tap target permanent an opponent controls.

Soul's Attendant

Whenever another creature enters, you may gain 1 life.

Felidar Guardian

When Felidar Guardian enters, you may exile another target permanent you control, then return that card to the battlefield under its owner's control.

Restoration Angel

Flash
Flying
When Restoration Angel enters, you may exile target non-Angel creature you control, then return that card to the battlefield under your control.

Soul Warden

Whenever another creature enters, you gain 1 life.

Sky Hussar

Flying
When Sky Hussar enters, untap all creatures you control.
Forecast — Tap two untapped white and/or blue creatures you control, Reveal Sky Hussar from your hand: Draw a card. (Activate only during your upkeep and only once each turn.)

Kiki-Jiki, Mirror Breaker

Haste
{T}: Create a token that's a copy of target nonlegendary creature you control, except it has haste. Sacrifice it at the beginning of the next end step.

Ajani's Welcome

Whenever a creature you control enters, you gain 1 life.

Runaway Steam-Kin

Whenever you cast a red spell, if Runaway Steam-Kin has fewer than three +1/+1 counters on it, put a +1/+1 counter on Runaway Steam-Kin.
Remove three +1/+1 counters from Runaway Steam-Kin: Add {R}{R}{R}.

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.

Timestream Navigator

Ascend (If you control ten or more permanents, you get the city's blessing for the rest of the game.)
{2}{U}{U}, {T}, Put Timestream Navigator on the bottom of its owner's library: Take an extra turn after this one. Activate only if you have the city's blessing.

Splinter Twin

Enchant creature
Enchanted creature has "{T}: Create a token that's a copy of this creature, except it has haste. Exile that token at the beginning of the next end step."

Impact Tremors

Whenever a creature you control enters, Impact Tremors deals 1 damage to each opponent.

Grinning Ignus

{R}, Return Grinning Ignus to its owner's hand: Add {C}{C}{R}. Activate only as a sorcery.

Zealous Conscripts

Haste
When Zealous Conscripts enters, gain control of target permanent until end of turn. Untap that permanent. It gains haste until end of turn.