msem-instigator

{G}, {T}: Add one mana of any color. When a creature enters this turn, if that mana was spent to cast it, it 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.)

Tortle Traveler enters tapped.
When Tortle Traveler enters, it 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.)

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.)

({T}: Add {W} or {U}.)
Tranquil Stream enters tapped.

As long as you control three or more artifacts, Treasury Master gets +2/+0 and has slipstream. (This creature can't be blocked as long as only creatures with slipstream are attacking.)

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

Hexproof
Whenever this creature transforms into Tsunami Titan or attacks, you may return target creature to its owner's hand.

Hexproof

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.

{1}{U}: Twilight Privateer voyages. (If it isn't blocking or targeted by another spell or ability, it phases out.)
Whenever Twilight Privateer phases in, up to one other target creature voyages.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
{1}{B}, Sacrifice a creature: Put a +1/+1 counter on Twisted Cultist.

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.

Tyrant of the Depths has hexproof as long as it's untapped.
Whenever you cycle Tyrant of the Depths or it attacks, tap target nonland permanent you don't control and put a stun counter on it.
Basic landcycling {1}{U} ({1}{U}, Discard this card: Search your library for a basic land card, reveal it, and put it into your hand, then shuffle.)

If a creature card would be put into an opponent's graveyard from anywhere, instead exile it, then copy it. You may cast the copy. (You still pay its costs, including color. Copies of permanents enter as tokens.)

Hexproof
When Uncharted Leviathan enters, target creature can't be blocked this turn.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)

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.

Whenever one or more creatures you control deal combat damage to a player, that player discards a card. Then that player may discard another card. If they don't, create two 2/1 black Nightmare creature tokens with “This creature can't block.”
Fleeting {1}{B}{B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)

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

First mate—There are four or fewer different mana costs among nonland cards in your starting deck. (If this card outside the game is your chosen first mate, you may swap it with a card in your opening hand.)
First strike
Each spell your opponents cast that would cost less than three mana to cast costs {1} more to cast.

Whenever Veiled Charmer attacks, target creature blocks it this turn if able.

Veiled Merfolk can't be blocked by creatures with greater power.
When Veiled Merfolk enters, mill a card.

{T}: Add {C}. Spend this mana only to cast an instant or sorcery spell or activate an ability.
{2}{U}: Move a counter from target creature onto another target creature.

Defender
Venerable Loremaster can attack as though it didn't have defender if it's in a pack. (Two or more creatures you control with equal power are in a pack.)