Pongify

Destroy target creature. It can't be regenerated. Its controller creates a 3/3 green Ape creature token.

Cursecatcher

Sacrifice this creature: Counter target instant or sorcery spell unless its controller pays {1}.

Counterspell

Rishadan Dockhand

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
{1}, {T}: Tap target land.

Spreading Seas

Enchant land
When this Aura enters, draw a card.
Enchanted land is an Island.

Sea's Claim

Enchant land (Target a land as you cast this. This card enters attached to that land.)
Enchanted land is an Island.

Silvergill Adept

As an additional cost to cast this spell, reveal a Merfolk card from your hand or pay {3}.
When this creature enters, draw a card.

Tide Shaper

Kicker {1} (You may pay an additional {1} as you cast this spell.)
When this creature enters, if it was kicked, target land becomes an Island for as long as this creature remains on the battlefield.
This creature gets +1/+1 as long as an opponent controls an Island.

Grafdigger's Cage

Creature cards in graveyards and libraries can't enter the battlefield.
Players can't cast spells from graveyards or libraries.

Relic of Progenitus

{T}: Target player exiles a card from their graveyard.
{1}, Exile this artifact: Exile all graveyards. Draw a card.

Master of the Pearl Trident

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Merrow Reejerey

Other Merfolk creatures you control get +1/+1.
Whenever you cast a Merfolk spell, you may tap or untap target permanent.

Merfolk Trickster

Flash
When this creature enters, tap target creature an opponent controls. It loses all abilities until end of turn.

Lord of Atlantis

Other Merfolk get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)