Whenever a face-down creature you control deals combat damage to an opponent, you may exile it, then cast it without paying its mana cost.
{2}{R}{R}: Put a card from your hand onto the battlefield face down.

{T}: Each opponent mills a card.
{3}, {T}: Exile three target cards from an opponent's graveyard. Draw a card.

Devoid
Target creature you control gets +2/+2 until end of turn if it's colorless. That creature fights target creature you don't control.

Non-Human creatures you control get +1/+0.

Acid Hydra enters the battlefield with X +1/+1 counters on it.
Drop — When Acid Hydra enters the battlefield, search your library for a creature card with mana value X or less, put it onto the battlefield, then shuffle. Then return that creature to your hand unless you didn't cast any creature spells during your last turn.

This spell costs {1} less to cast if you've cast another spell that targeted a creature this turn.
Target creature you control gets +3/+0 and gains first strike until end of turn.

Target creature gets +2/+2 and gains indestructible until end of turn.

Adaptive Blade enters the battlefield with your choice of two different counters on it from among menace, haste, first strike, or trample.
Equipped creature gets +1/+0.
Keyword counters on Adaptive Blade apply to equipped creature.
Equip {1}

Return up to two target creature cards from your graveyard to your hand. You lose 2 life.

Each spell you cast with flash or haste costs {1} less to cast.
Each creature you control with flash or haste gets +1/+0.

Target creature you control gets +4/+3 until end of turn.

Aerial Sleuth has flying as long as it's attacking.

Whenever Aether Conductor attacks, put a charge counter on it, then you may put a permanent card with mana value equal to or less than the number of charge counters on Aether Conductor from your hand onto the battlefield tapped and attacking.

Target creature you control gets +X/+X, gains trample, and becomes a legendary Rebel until end of turn, where X is its power. Sacrifice that creature at the beginning of the next end step.

Flash
When Agent of Deception enters the battlefield, counter target creature or planeswalker spell unless its controller pays {X}, where X is your devotion to blue.

Target creature you control deals damage equal to its power to target creature you don't control.

When Aglu's Ghostwarden enters the battlefield, surveil 2.

{X}{U}{B}, {T}: Exile the top X cards of target opponent's library. You may cast a nonland card from among them with mana value X or less without paying its mana cost. Put the rest on the bottom of that library in a random order.

Target player reveals their hand. You choose a nonland card from it or a card from their graveyard. Exile that card. You lose 1 life.

All creatures get -1/-1 until end of turn.
Undercover

At the beginning of each opponent's upkeep, that player may pay life equal to the number of pain counters they have plus 1. If they don't, look at their hand, choose a card from it, and exile that card. If they do, that player gets a pain counter.
You may cast cards exiled with Alehana, and mana of any type can be spent to cast them.

Alehana's Eyes has +1/+1 and can't be blocked as long as you've cast an instant or sorcery spell with mana value 4 or greater this turn.

Create a token that's a copy of target creature you control, except it's a 1/1. When you do, pay for and choose any number.
+{C} — That token becomes a copy of a different target creature.
+{W} — You gain life equal to the number of creatures you control.
+{B} — Each player sacrifices a creature.

Counter up to one target spell unless its controller pays {1}. Your opponents can't cast spells this turn.

{3}{B}: Alley Shade gets +2/+2 and gains lifelink until end of turn.