msem-instigator

Vigilance
When Tortle Guard enters, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Whenever Tortle Veteran or another Turtle you control enters, that creature explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

Haste
Whenever Tortle Waymaker becomes blocked, it explores. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then you may put that card in your graveyard.)

Mark (Whenever this creature attacks, mark target creature until end of turn.)
Whenever a marked creature dies, target player discards a card.

Whenever Toxic Lurker attacks, you may have target creature defending player controls get -2/-1 until end of turn.

Vision (Whenever this or another creature you control enters, reveal the top card of your library. Then you may put that card into your graveyard.)
Whenever Tracker of Wild Places envisions a creature card, you may untap a permanent you control.

Rapport — Whenever Trae, Serpent's Grasp and at least one other creature you control become tapped, each player draws a card. Then Trae, Serpent's Grasp deals 2 damage to each opponent with four or more cards in hand.

Land creatures you control get +1/+1 and have vigilance.
Enchantment creatures you control get +1/+1 and have lifelink.
Artifact creatures you control get +1/+1 and have flying.

When Transcended Explorer enters, if you control another enchantment, draw a card.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)
{1}{W}, Sacrifice a creature or enchantment: Scry 1, then draw a card. Activate only if Trauma Surgeon has a +1/+1 counter on it.

When Traversing Bulkvine enters, search your library for three land cards with different names and reveal them. An opponent chooses one of them. Put that card into your graveyard and the rest into your hand, then shuffle.

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever you cast your first instant or sorcery spell each turn, copy that spell. You may choose new targets for the copies.

Trial 4 (As you cast this spell, an opponent of your choice may pay 4 life. If they don't, they fail the trial.)
When Treacherous Devil enters, if an opponent passed the trial, that player gains control of Treacherous Devil.

When Treacherous Warden enters, search your library for a card, put into your graveyard, then shuffle.
Whenever a creature enters from your graveyard, each opponent loses 2 life and you gain 2 life.

Haste
When Treasure Haulers enters, you may salvage an artifact card. (Exile one from your graveyard until this creature leaves the battlefield. Whenever this creature deals combat damage to a player, put a card it salvaged into your hand.)

Vigilance, reach
When Treeline Archer enters, mill four cards.
{5}{G}: Transform Treeline Archer. Activate only if it's attacking.

Whenever another nontoken creature you control enters, that creature gains haste until end of turn. Then Tremor Seer deals 1 damage to any target.

When Trench Breaker enters, put target creature on top of its owner's library.

When Trenchbottom Mariners enters, you lose 3 life, mill three cards, and create a Treasure token.
Whenever Trenchbottom Mariners attacks, you may cast a permanent card from your graveyard by sacrificing another artifact in addition to paying its costs.

Whenever you cast a spell from your graveyard, you may return target nonland permanent to its owner's hand.
Sacrifice a Clue: Return Trespasser of the Gallows from your graveyard to your hand.

{4}{G}{G}, Exile six other cards from your graveyard: Return Trifold Atrocity from your graveyard to the battlefield with three +1/+1 counters on it. Activate only as a sorcery.

Triplicate Terror can't be blocked unless defending player pays {3}. (They pay it once each combat.)

{2}, Sacrifice another creature: 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.”)

Menace
{3}{R}: Transform Troublesome Throng. Activate only if it's attacking.

Vigilance
Assault {G} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)