msem-instigator

Whenever Memoria Thief deals combat damage to a player, look at that player's hand. You may choose a nonland card from it. If you do, that player reveals the chosen card, puts it on the bottom of their library, then draws a card.

When Obscure Apostle enters, create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)
Whenever you sacrifice an Idol, Obscure Apostle can't be blocked this turn.

Flying
Whenever an opponent casts a spell, counter it unless that player pays {1} for each spell cast before it this turn.

Flash
Flying
Whenever you cast another spell with flash or an instant, conjure a card named Data-Point into your hand. (It's a sorcery that costs {2} with “Draw a card.”)

Flash
Flying
When Powder Scatterer enters, exile target noncreature spell. Its controller draws a card.

Flash
When Predatory Psion enters, switch target creature's power and toughness until end of turn.

When Saddled Scuttleback enters, you may search your library for a Crab, Fish, Jellyfish, Kraken, Leviathan, Merfolk, Nautilus, Octopus, Otter, Oyster, Pirate, Seal, Serpent, Shark, Shrimp, Sponge, Squid, Starfish, Trilobite, Turtle, Whale, or Island card, reveal it, put it into your hand, then shuffle.

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.)

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.