msem-instigator

When The Lonely Prophet enters the battlefield, exile a card you own from outside the game face down.
Whenever The Lonely Prophet deals combat damage, put the exiled card into your hand.

Convoke (Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Protection from noncreatures
When The Magistrate enters the battlefield, judge each creature that convoked it. (Honor it by putting a +1/+1 counter on it or condemn it by sacrificing it to draw a card.)

When The Moon Rises enters the battlefield, exile all opponent's graveyards.
At the beginning of your upkeep, each opponent loses 1 life.

When The Raven Queen of Fretport enters the battlefield, put a -1/-1 counter on each of any number of target creatures.
{2}{U}{B}: Proliferate. (Choose any number of permanents and/or players with counters on them, then give each another counter of each kind already there.)

(Choose only one work as a sorcery to add its ability.)
When The Royal Vignettes enters the battlefield, exile target instant or sorcery card from your graveyard. Until your next turn, you may cast that card.
{1}{U}: Ju, the Artisan
Whenever you cast an instant spell, draw a card, then discard a card.
{1}{R}: Lan, the Poet
Whenever you cast a sorcery spell, create a 1/1 red Bard creature token with “This creature can't block.”

Trample
When The Shepherd of Nowhere enters the battlefield, each player sacrifices each other nontoken creature they control.
Whenever another nontoken creature you control dies, create a token that's a copy of that creature.

Deathtouch
When The Terror of Shady Cove enters the battlefield, target opponent sacrifices an artifact. If they can't, create a 3/3 green Slug creature token with deathtouch. Then repeat this process for an enchantment and a nonbasic land.

When The Third Breach enters the battlefield, discard your hand, then draw seven cards.
Creatures you control get +3/+3, have haste, and are Horrors in addition to their other types.
At the beginning of each end step, sacrifice each creature you control.

Flying, haste
Thermal Surger can't block.
When Thermal Surger enters the battlefield, you may sacrifice a land or exile a land card from your graveyard. If you do, put a +1/+1 counter on Thermal Surger.

When Thespian's Page enters the battlefield, return target instant or sorcery card from your graveyard to your hand.

When Theurgist of the Depths enters the battlefield, for each card type, you may exile a card of that type from your graveyard. Target opponent separates those cards into two piles. Put all cards from the pile of your choice into your hand.
Recite {B}{G}{U} (You may cast this creature from your graveyard for its recite cost. If you do, exile it when it enters the battlefield or if it would leave the battlefield.)

When Third Legion Rider enters the battlefield, if you control an embattled creature, tap target creature an opponent controls. It doesn't untap during its controller's next untap step. (A creature is embattled if it has a counter on it or has a permanent you control attached to it.)

When Thorn Tender enters the battlefield, rouse 1, then put a +1/+1 counter on each creature you control. (Put a +1/+1 counter on a permanent you control. If it isn't a creature, tap it and it becomes a 0/0 creature in addition to its other types.)

Flash
When Thornweald Cavalier enters the battlefield, target creature gets +1/+1 until end of turn. Put a reach counter on it.

Thornwood Falls enters the battlefield tapped.
When Thornwood Falls enters the battlefield, you gain 1 life.
{T}: Add {G} or {U}.

(Choose only one work as a sorcery to add its ability.)
When Those Lost to the Waves enters the battlefield, search your library for an artifact card with mana value 3 or less, reveal it, put it into your hand, then shuffle.
{1}{W}: Fight the Current
Equipped creatures you control get +1/+1.
{3}{U}: Soul of the Sea
At the beginning of combat on your turn, you may have target noncreature artifact become a 3/3 artifact creature until end of turn.

When Thought Paradigm enters the battlefield, conjure a card named Data-Point into your hand. (It's a sorcery that costs {2} with “Draw a card.”)
Whenever you cast a sorcery spell, target player mills three cards.

When Thoughtgrinder enters the battlefield, each player discards a card.
Unearth {1}{U}{B} ({1}{U}{B}: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the end step or if it would leave the battlefield. Unearth only as a sorcery.)

When Thoughtslick Engineer enters the battlefield, each player discards a card at random.

Flash
When Threadripper Savant enters the battlefield, put a stun counter on up to one target creature.

Flash
When Threadspinner enters the battlefield, untap each enchanted permanent you control. Enchanted creatures you control get +1/+1 until end of turn.

When Three Seas Squire enters the battlefield, target creature or Vehicle gets +2/+1 until end of turn.

When Threshing Sundew enters the battlefield, merge up to one target creature card in your graveyard under it. Threshing Sundew has that card's abilities.
When Threshing Sundew dies, return all other cards merged with it to their owner's hand.

Trample, haste
Thriving Hydra enters the battlefield with four +1/+1 counters on it.
When Thriving Hydra enters the battlefield, for each other creature you control, put X +1/+1 counters on that creature, where X is four minus its power.
Whenever Thriving Hydra attacks, double the number of +1/+1 counters on each creature you control.

When Through the Dust enters the battlefield, it becomes an Aura with enchant creature. Reveal the top seven cards of your library, put a creature card revealed this way onto the battlefield and attach Through the Dust to it. Then put the rest on the bottom of your library in any order. When Through the Dust is no longer on the battlefield, enchanted creature's controller sacrifices it.
Mirage {1}{U}{U}{U}