msem-instigator

Flash
When Scorned Arcanist enters, until end of turn if a creature would enter the battlefield, instead exile that creature and that creature's controller amasses Soldiers X, where X is that creature's mana value. (They put X +1/+1 counters on an Army they control. It's also a Soldier. If they don't control an Army, they create a 0/0 black Soldier Army creature token first.)

Flash
Flying
When Spectrum Archmage enters, return target spell to its owner's hand, then abound. (To abound, you may cast a spell that costs less than this from your hand without paying its mana cost.)

Heroic — Whenever you cast a spell that targets Thassa's Shoreguards, tap target creature an opponent controls. It doesn't untap during its controller's next untap step.
Whenever Thassa's Shoreguards deals combat damage to a player, draw a card.

Respawn (When this creature dies, if it attacked this turn, return it to its owner's hand.)
When Tutorial Boss enters, create a 1/1 blue Pirate creature token.

Flash
Flying
Sacrifice Vengeful Messenger: Counter target noncreature spell unless its controller pays {2}.

Flying
Wailing Soul has hexproof as long as it's not attacking or blocking.

Flash
When Wiretapper enters, choose one —
• Exile target spell. For as long as that card remains exiled, its owner may play it. A spell cast this way costs {2} more to cast.
• Create a copy of the card with target spell's name. You may cast the copy. (You still pay its costs.)

When Acolyte of Griselbrand dies, you draw a card and you lose 1 life.
Foreshadow {4}{B} (You may cast this spell for its foreshadow cost. If you do, also trigger its “dies” abilities when it enters.)

Deception {1}{B}
When Aguri, Shadow of Doubt enters, target player reveals their hand. You choose a nonland card from it. That player discards that card. You lose 2 life.

Flying, lifelink

Deathtouch
When Bitterheart Bokor leaves the battlefield, you may search your library for a Curse card, reveal it, put it into your hand, then shuffle.

Haste
When Blighted Dullahan dies, you draw a card and you lose 1 life.

When Breaker of Secrets enters, each player mills three cards from each of their libraries.
Recite {U} (You may cast this creature from your graveyard for its recite cost. If you do, exile it when it enters or if it would leave the battlefield.)

Glorious end — When Brutal Champion dies, if it was tapped, each opponent sacrifices a creature.

Whenever a creature enters, if it entered from a graveyard or a player cast it from a graveyard, return Ceaseless Stalker from your graveyard to the battlefield at the beginning of the next end step.

Flying

Haste
Coldhearted Explorer can't block.

{1}{B}, Discard Crusher Claw: Target creature gains winnow until end of turn. (Damage dealt to it also causes its controller to lose that much life.)

Dawson's Marauder can't block.
When Dawson's Marauder attacks, it can't be blocked this turn unless defending player sacrifices an artifact.

Menace
Whenever Despair Scavenger attacks, you may have target creature get -1/-1 until end of turn.

{1}, Sacrifice a creature: Desperate Forsaken gets +1/+1 until end of turn. You gain 1 life.

Haste
Fleeting {B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)

Whenever Devourer of Legacy deals combat damage to a player, you may sacrifice it and choose a nonland card name. If you do, search that player's graveyard, hand, and library for any number of cards with that name and exile them. That player shuffles, then draws a card for each card exiled from their hand this way.

Lionize {2}{B} ({2}{B}, Exile this card from your hand: Put a +1/+1 counter on target creature and it gains the following ability until end of turn. You may cast this card on a later turn from exile.)
Lifelink

When Exiled Necromancer enters, return target permanent card with mana value 1 or less from your graveyard to the battlefield.