Void Mirror

Whenever a player casts a spell, if no colored mana was spent to cast it, counter that spell.

Dovin's Veto

This spell can't be countered.
Counter target noncreature spell.

Aether Vial

At the beginning of your upkeep, you may put a charge counter on this artifact.
{T}: You may put a creature card with mana value equal to the number of charge counters on this artifact from your hand onto the battlefield.

Plains

Tempest Caller

When this creature enters, tap all creatures target opponent controls.

Subtlety

Flash
Flying
When this creature enters, choose up to one target creature spell or planeswalker spell. Its owner puts it on their choice of the top or bottom of their library.
Evoke—Exile a blue card from your hand.

Lord of Atlantis

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

Mana Leak

Counter target spell unless its controller pays {3}.

Cavern of Souls

As this land enters, choose a creature type.
{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

Seachrome Coast

This land enters tapped unless you control two or fewer other lands.
{T}: Add {W} or {U}.

Island

Svyelun of Sea and Sky

Svyelun has indestructible as long as you control at least two other Merfolk.
Whenever Svyelun attacks, draw a card.
Other Merfolk you control have ward {1}. (Whenever another Merfolk you control becomes the target of a spell or ability an opponent controls, counter it unless that player pays {1}.)

Path to Exile

Exile target creature. Its controller may search their library for a basic land card, put that card onto the battlefield tapped, then shuffle.

Field of Ruin

{T}: Add {C}.
{2}, {T}, Sacrifice this land: Destroy target nonbasic land an opponent controls. Each player searches their library for a basic land card, puts it onto the battlefield, then shuffles.

Meddling Mage

As this creature enters, choose a nonland card name.
Spells with the chosen name can't be cast.

Displacement Wave

Return all nonland permanents with mana value X or less to their owners' hands.

Cursecatcher

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

Mutavault

{T}: Add {C}.
{1}: This land becomes a 2/2 creature with all creature types until end of turn. It's still a land.

Unsettled Mariner

Changeling (This card is every creature type.)
Whenever you or a permanent you control becomes the target of a spell or ability an opponent controls, counter that spell or ability unless its controller pays {1}.

Ghost Quarter

{T}: Add {C}.
{T}, Sacrifice this land: Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.

Thada Adel, Acquisitor

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Thada Adel deals combat damage to a player, search that player's library for an artifact card and exile it. Then that player shuffles. Until end of turn, you may play that card.

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.

Wanderwine Hub

As this land enters, you may reveal a Merfolk card from your hand. If you don't, this land enters tapped.
{T}: Add {W} or {U}.

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.

Vodalian Hexcatcher

Flash
Other Merfolk you control get +1/+1.
Sacrifice a Merfolk: Counter target noncreature spell unless its controller pays {1}.

Merfolk Trickster

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

Spreading Seas

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

Cryptic Command

Choose two —
• Counter target spell.
• Return target permanent to its owner's hand.
• Tap all creatures your opponents control.
• Draw a card.

Secluded Courtyard

As this land enters, choose a creature type.
{T}: Add {C}.
{T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type or activate an ability of a creature source of the chosen type.