msem-instigator

Counter target activated or triggered ability.
Metamorph {U} (You may cast this card face down as a 2/2 creature for {3}. Turn it face up and cast it at any time for its metamorph cost.)

Put target nonland permanent on top of its owner's library.

Target player mills two cards.
Draw a card.
Coalesce {6}{U} (You may cast this from your graveyard for {6}{U}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)

Changeling (This card is every creature type.)
Watcher of the Polyps has all names of creature cards in addition to its name as long as it's on the battlefield.
Watcher of the Polyps gets +X/+X, where X is the greatest number of creatures you control with the same name.

Each other creature you control that's a Merfolk or an Elemental gets +1/+1.
Whenever you cast an instant, sorcery, or Merfolk spell, you may pay {1}. If you do, create a 1/0 blue Elemental creature token.

Counter target spell unless its controller pays {2}. If there are three or more counters among permanents you control, counter that spell instead.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
{3}{U}: Target creature gains slipstream until end of turn.

Target player mills four cards.
Dominance — That player mills eight cards instead if you control the creature with the greatest power or tied for the greatest power.

Create two Idol tokens, then counter target spell unless its controller pays {1} for each Idol you control. (They're artifacts with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)

Counter up to one target spell unless its controller pays {2}. Until end of turn, you may cast spells as though they had flash.

Flying
Cycling {2} ({2}, Discard this card: Draw a card.)

Create an 8/8 blue Kraken creature token.
{2}{U}, Discard Wealth of the Flood: Create a Treasure token. Scry 2.

When Wealthy Merchants enters, create a Treasure token. (It's an artifact with “{T}, Sacrifice this artifact: Add one mana of any color.”)

Creatures you control get +0/+2 and gain hexproof until end of turn.

Whenever Weaver of Lore attacks, you may reveal your hand. Weaver of Lore gets +2/+2 until end of turn if you reveal two or more blue cards this way.

When Weaver of Mists enters, substantiate target card in your graveyard. (Put it onto the battlefield face down as a 2/2 creature. You may turn it face up and cast it at any time for its mana cost if it isn't a creature.)
Spells you cast from anywhere other than your hand cost {1} less to cast.

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 Weaver of the Winds envisions an instant or sorcery card, draw a card.

Counter target spell if you have more cards in your graveyard than that spell's controller.

Flying
Welkin Tern can block only creatures with flying.

Wharf Lumper can't be blocked by creatures with greater power.
Whenever Wharf Lumper attacks, mill a card. If that card was an instant or sorcery, Wharf Lumper gets +3/+0 until end of turn.

Enchant permanent
Whenever enchanted permanent becomes untapped, its controller gets a poison counter.

Draw two cards, then discard two cards. Investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)

Whenever you draw a card, target player mills two cards.
Whenever you cast a blue spell, scry 1, then you may reveal the top card of your library. If it's an instant or sorcery card, draw a card.

Return target nonland permanent to its owner's hand. Scry 1.

Draw two cards, then discard two cards if it's a combat step.
Quickdraw {U} (When a creature you control blocks or becomes blocked, you may cast this spell for {U}.)