Case of the Gateway Express

When this Case enters, choose target creature you don't control. Each creature you control deals 1 damage to that creature.
To solve — Three or more creatures attacked this turn. (If unsolved, solve at the beginning of your end step.)
Solved — Creatures you control get +1/+0.

Seal from Existence

Ward {3} (Whenever this enchantment becomes the target of a spell or ability an opponent controls, counter it unless that player pays {3}.)
When Seal from Existence enters, exile target nonland permanent an opponent controls until Seal from Existence leaves the battlefield.

Plains

Baird, Steward of Argive

Vigilance
Creatures can't attack you or planeswalkers you control unless their controller pays {1} for each of those creatures.

Goldnight Commander

Whenever another creature you control enters, creatures you control get +1/+1 until end of turn.

Guardian of the Great Door

As an additional cost to cast this spell, tap four untapped artifacts, creatures, and/or lands you control.
Flying

Have

Evangel of Heliod

When Evangel of Heliod enters, create a number of 1/1 white Soldier creature tokens equal to your devotion to white. (Each {W} in the mana costs of permanents you control counts toward your devotion to white.)

Orphans of the Wheat

Whenever Orphans of the Wheat attacks, tap any number of untapped creatures you control. Orphans of the Wheat gets +1/+1 until end of turn for each creature tapped this way.

Forbidding Spirit

When Forbidding Spirit enters, until your next turn, creatures can't attack you or planeswalkers you control unless their controller pays {2} for each of those creatures.

Savior of the Small

Survival — At the beginning of your second main phase, if Savior of the Small is tapped, return target creature card with mana value 3 or less from your graveyard to your hand.

Veteran Survivor

Survival — At the beginning of your second main phase, if Veteran Survivor is tapped, exile up to one target card from a graveyard.
As long as there are three or more cards exiled with Veteran Survivor, it gets +3/+3 and has hexproof. (It can't be the target of spells or abilities your opponents control.)

Court Street Denizen

Whenever another white creature you control enters, tap target creature an opponent controls.

Kor Sanctifiers

Kicker {W} (You may pay an additional {W} as you cast this spell.)
When Kor Sanctifiers enters, if it was kicked, destroy target artifact or enchantment.

Conclave Phalanx

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
When Conclave Phalanx enters, you gain 1 life for each creature you control.

Mage's Attendant

When Mage's Attendant enters, create a 1/1 blue Wizard creature token with "{1}, Sacrifice this creature: Counter target noncreature spell unless its controller pays {1}."

Resolute Reinforcements

Flash (You may cast this spell any time you could cast an instant.)
When this creature enters, create a 1/1 white Soldier creature token.

Lionheart Glimmer

Ward {2} (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
Whenever you attack, creatures you control get +1/+1 until end of turn.

Rumor Gatherer

Alliance — Whenever another creature you control enters, scry 1. If this is the second time this ability has resolved this turn, draw a card instead.

Malamet War Scribe

When Malamet War Scribe enters, creatures you control get +2/+1 until end of turn.