msem-instigator

Whenever a creature dealt damage by Truth Extractor this turn dies, investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)

Flying
Whenever you cast your first noncreature spell each turn, create a Treasure token. (It's an artifact with “{T}, Sacrifice this artifact: Add one mana of any color.”)

Flash
When Tsang Jin, Master of the Jade Sea enters, put target land card from a graveyard onto the battlefield under your control.

Trample, hexproof
Renown 4 (When this creature deals combat damage to a player, if it isn't renowned, put four +1/+1 counters on it and it becomes renowned.)
{1}{G}: Target creature blocks Tsolmon, the Huntress this turn if able.

Hexproof

Flying, lifelink
Sacrifice a creature: Until end of turn, Turned Watcher gets +1/+0 and gains “Whenever Turned Watcher 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.

Whenever Twilight Walker perseveres, target creature doesn't untap during its controller's next untap step. (A creature perseveres when it survives damage.)

Gestalt (When this or another creature enters under your control, you may merge it with the one in play, keeping counters and attachments. They become the permanent on top plus all abilities from under it.)
Other creatures you control have base power and toughness equal to this creature's base power and toughness.

When Twinforged Golem enters, create a 3/3 colorless Golem artifact creature token.

Menace
Chosen one ({W}{U}{B}{R}{G}: Transform this permanent.)

Whenever Twitchy Pyromancer enters or attacks, it deals 1 damage to any target. Whenever a creature dealt damage this way dies this turn, upgrade Twitchy Pyromancer. (Give it one of the listed counters it doesn't already have.)
• +1/+1
• Menace
• Prowess

Sacrifice a creature: Two-Faced Aristocrat gains indestructible until end of turn.
Attain {4}{B} ({4}{B}: Transform this. Spend at least three colors of mana to activate this, and only as a sorcery.)

Creatures you control dying cause abilities of that creature to trigger as though that creature entered.
{2}{W}{B}, Sacrifice another creature: Destroy target nonland permanent.

Flying
{1}{R}{R}{R}: Return Umbral Hunter from your graveyard to your hand. If it's the declare attackers step, up to one target attacking creature gets +3/+1 until end of turn.

Whenever you copy an instant or sorcery spell, you may pay {1}{U}. If you do, draw a card.
Whenever you cast an instant or sorcery spell, you may pay {1}{R}. If you do, Unbound Researchers deals 2 damage to target player or planeswalker.

Whenever Unbudging Badger attacks, if its power is 4 or greater, it gains trample until end of turn.

Whenever Uncanny Allocator enters or attacks, give any number of permanents and/or players with counters on them another counter of each kind already there.

When Undead Servant enters, create a 2/2 black Zombie creature token for each card named Undead Servant in your graveyard.

When this creature enters, search your library for a creature card with mana value 1 or less, reveal it, put it into your hand, then shuffle.
Arise {2}{W} ({2}{W}, Exile this from your graveyard as a sorcery: Create a black token copy of it that's also a Zombie and can't block.)

Undine Coven gets +1/+1 for each other Merfolk you control.
Whenever you cast a noncreature spell, Undine Coven deals 1 damage to target player or planeswalker.

When Undying Watcher enters, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)
Sacrifice another creature: Undying Watcher gains indestructible until end of turn.

Other creatures you control get +1/+0.
At the beginning of combat on your turn, create a 1/1 red Human creature token with haste.

Flying
Whenever you cast a spell from your graveyard, return Unliving Swarm from your graveyard to your hand.

Trample
Whenever Unruly Cannoneer crews a Vehicle, that Vehicle gets +1/+1 and gains trample until end of turn.