msem-instigator

When Edina's Soulsinger enters, create a 1/1 blue Jellyfish enchantment creature token with slipstream. (It can't be blocked as long as only creatures with slipstream are attacking.)

Counter target creature spell. You may cast a copy of that spell. (You still pay its costs. The copy becomes a token.)

Enchant creature
You control enchanted creature.
Enchanted creature gets +1/+1 and has slipstream. (This creature can't be blocked as long as only creatures with slipstream are attacking.)

Pressure {X}{U}{U} (You may cast this spell for its pressure cost if seven or more cards are in your graveyard.)
Look at the top card of your library. You may put that card into your graveyard.
Draw a card. If this spell was cast for its pressure cost, draw X cards instead.

Draw cards equal to the number of cards in your hand, then discard three cards.
Cycling {U} ({U}, Discard this card: Draw a card.)

Pressure {1}{U}{U} (You may cast this spell for its pressure cost if seven or more cards are in your graveyard.)
Draw three cards.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Whenever Happy Manta deals combat damage to a player, draw a card for each card type among spells you've cast this turn.

Pressure {1}{U} (You may cast this spell for its pressure cost if seven or more cards are in your graveyard.)
Exile up to two target nonland permanents you control, then return them to the battlefield under their owner's control.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Pressure {U} (You may cast this spell for its pressure cost if there are seven or more cards in your graveyard.)

Return up to two target creatures to their owner's hands.
Cycling {2} ({2}, Discard this card: Draw a card.)

This spell costs {2} less to cast if you've cast another spell this turn.
Draw two cards, then put a card from your hand on top of your library.

Other Merfolk you control get +1/+1.
{1}{U}: Target Merfolk gains slipstream until end of turn. (Creatures with slipstream can't be blocked as long as only creatures with slipstream are attacking.)

Flash (You may cast this spell any time you could cast an instant.)
When Majestic Hippocamp enters, target creature gets -2/-0 until end of turn.

When Master of the Abyss enters, draw two cards, then discard a card.
{2}{U}{U}, {T}, Discard two cards: Create an X/X black Horror creature token, where X is the number of cards in your graveyard.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
You may cast the first spell that's an enchantment, instant, or sorcery with mana value 1 you cast each turn without paying its mana cost.

When Mystic Mackerel enters, you may have target player mill two cards.
{3}{U}, Sacrifice Mystic Mackerel: Return target enchantment, instant, or sorcery card from your graveyard to your hand.

Whenever you draw a card, put a foreshadow counter on Ominous Seas.
Remove eight foreshadow counters from Ominous Seas: Create an 8/8 blue Kraken creature token.
Cycling {2} ({2}, Discard this card: Draw a card.)

{8}: Create eight 1/1 blue Tentacle creature tokens.
{8}: Counter target spell unless its controller pays {8}.
{8}: Draw eight cards.

Look at the top five cards of your library. You may reveal an enchantment, instant, or sorcery card from among them and put it into your hand. Put the rest on the bottom of your library in a random order.

Defender
Whenever you cast an enchantment, instant, or sorcery spell, scry 1 if it's the first time this ability has resolved this turn. If it's the second time, Prismscale Piranhas can attack this turn as though it didn't have defender. If it's the third time, whenever Prismscale Piranhas deals combat damage to a player this turn, draw a card.

Concord — When Riptide Mystic enters, if there are four or more card types among permanents you control, you may return target nonland permanent to its owner's hand.

When Seafloor Casket enters and when you sacrifice it, target player mills two cards.
{1}{U}, Sacrifice Seafloor Casket: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

{T}: Scry 1.

Choose one —
• Counter target spell unless its controller pays {4}.
• Tap up to two target creatures. They don't untap during their controller's next untap step.

Pressure—{2}{U}{U}, Discard your hand. (You may cast this spell for its pressure cost if seven or more cards are in your graveyard.)
Reveal the top seven cards of your library. Put any number of noncreature, nonland cards from among them into your hand and the rest into your graveyard.