msem-instigator

{5}{U}{U}: Thassa's Tidebringer can't be blocked this turn. This ability costs {1} less to activate for each creature you control with a counter on it.

Menace
When The Anguilahydra enters, draw X cards and put X +1/+1 counters on The Anguilahydra, where X is the number of enchantments you control.

Flying
Spirited (When this creature enters, you may activate one of its activated abilities without paying its costs.)
Harmony — Draw three cards.
{2}{G}{G}: Harmonize. (Trigger up to one harmony of a permanent you control.)

Enspirit (As you cast this spell, you may exile a nonenchantment creature you control until this leaves the battlefield or fails to resolve. Reduce this spell's cost by the exiled card's mana value.)
At the beginning of your upkeep, create a token that's a copy of the enspirited card. It gains haste until end of turn. Exile it at the beginning of the next end step.

When Theurgist of the Depths enters, 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 or if it would leave the battlefield.)

Whenever a creature you control phases in, it gets +2/+2 until end of turn.

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

Whenever Thundering Towershell attacks, return a creature card with mana value 5 or less from a graveyard to the battlefield under your control. Trigger a death knell of that creature.

Flash
Menagerie — When Tidal Trickster enters, if there are five or more creature types among creatures you control, you may search your library for a card named Tidal Trickster, reveal it, put it into your hand, then shuffle.

Whenever Tide Crasher attacks, if you control another Merfolk, tap target creature defending player controls.

Experience (Whenever this creature attacks, if it has fewer than three +1/+1 counters on it, put a +1/+1 counter on it.)
When Tidepool Turtle has three or more +1/+1 counters on it, remove them all and transform it.

First mate — Each nonland card in your starting deck is an artifact, a Pirate, or has “artifact,” “Treasure,” or “Pirate” in its printed rules text. (If this card outside the game is your chosen first mate, you may swap it with a card in your opening hand.)
Whenever Tinbeard of the Brine attacks, each opponent loses X life and you gain X life, where X is the number of artifacts you control plus the number of Pirates you control.

Colossal 2—{6}{U} (You may cast this for its colossal cost to have it enter with two +1/+1 counters; or pay {6}{U} as a sorcery: Put two +1/+1 counters on it if it has none.)
When Titanic Fathomclaw's colossal ability puts one or more counters on it, return target instant or sorcery card from your graveyard to your hand.

Whenever Titanic Hardshell deals combat damage to a player, draw a card.
{2}{G}: Titanic Hardshell gains trample until end of turn.

Tizzanier Tidehunter can't be blocked as long as you control another Merfolk.

({T}: Add {U} or {R}.)
Torrid Geysers enters tapped unless you control two or more basic lands.

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.)

Trample
{4}{R}{R}: Tortle Tinkerer explores. This ability costs {1} less to activate for each artifact you control. (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.)

{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.)