msem-instigator

Majestic Nauticorn has first strike as long as it's attacking.
Cycling {W} ({W}, Discard this card: Draw a card.)

Whenever Merrow Legendmaker attacks, look at the top five cards of your library. You may reveal an Aura, Aurashaper, or Equipment card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

When Militant's Standard enters, draw a card if you control a Merfolk.
Equipped creature gets +2/+1.
Equip {2}

Choose one —
• Return target artifact or enchantment card from your graveyard to your hand.
• Return target creature card with mana value 2 or less from your graveyard to your hand.
Cycling {2} ({2}, Discard this card: Draw a card.)

Enchant creature
Enchanted creature gets +3/+3.
Whenever enchanted creature attacks, you may put a creature card with mana value 2 or less from your hand or graveyard onto the battlefield tapped and attacking.

Concord — When Palace Servant enters, if there are four or more card types among permanents you control, create two 1/1 white Merfolk creature tokens.

When Pensive Muse enters, create a white Aura enchantment token named Seal's Love attached to target creature you control. The token has enchant creature and “Enchanted creatures you control get +1/+1.”

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
As long as you control three or more artifacts, Regal Seasnake has vigilance. (Attacking doesn't cause it to tap.)

Schoolmaster gets +1/+1 for each other Merfolk you control.
When Schoolmaster enters, create a 1/1 white Merfolk creature token.

When Seafloor Shrine enters and when you sacrifice it, create a 1/1 white Merfolk creature token.
{1}{W}, Sacrifice Seafloor Shrine: Draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

First strike, vigilance
When Subnautical Legionnaire enters, choose artifact, enchantment, or Merfolk. Exile target nonland permanent you don't control until you control no permanents of the chosen type.

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

Permanents you control gain indestructible until end of turn. Untap each creature you control.
Cycling {W} ({W}, Discard this card: Draw a card.)

For each player, you choose from among the permanents that player controls an artifact, a creature, an enchantment, and a planeswalker. Then each player sacrifices all other nonland permanents they control.

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 Trident of Benediction enters, create two 1/1 white Merfolk creature tokens.
Equipped creature has vigilance and “Whenever this creature attacks or blocks, creatures you control get +1/+1 until end of turn.”
Equip {2}

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Sacrifice Bonetooth Darter: Counter target instant, planeswalker, or sorcery spell unless its controller pays {1}.

Whenever you draw a card, you may have target player mill two cards.

Whenever you cast an enchantment, instant, or sorcery spell, 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.)

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Concord — As long as there are four or more card types among permanents you control, Bubbledome Waveblade gets +1/+1.

When Bubblespout Sentry enters, look at the top card of your library. You may put it into your graveyard.

Concord — When Coral Bruiser enters, if there are four or more card types among permanents you control, put target creature you don't control into its owner's library third from the top.

Enchant creature
Concord — When Coral Guidance enters, if there are four or more card types among permanents you control, draw a card.
Enchanted creature doesn't untap during its controller's untap step.

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
Cycling {U} ({U}, Discard this card: Draw a card.)

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