Counterspell

Dream Thrush

Flying
{T}: Target land becomes the basic land type of your choice until end of turn.

Startling Development

Until end of turn, target creature becomes a blue Serpent with base power and toughness 4/4.
Cycling {1} ({1}, Discard this card: Draw a card.)

Alchemist's Retrieval

Cleave {1}{U} (You may cast this spell for its cleave cost. If you do, remove the words in square brackets.)
Return target nonland permanent [you control] to its owner's hand.

Remote Isle

Remote Isle enters tapped.
{T}: Add {U}.
Cycling {2} ({2}, Discard this card: Draw a card.)

Shimmer of Possibility

Look at the top four cards of your library. Put one of them into your hand and the rest on the bottom of your library in a random order.

Coral Merfolk

Mystic Sanctuary

({T}: Add {U}.)
Mystic Sanctuary enters tapped unless you control three or more other Islands.
When Mystic Sanctuary enters untapped, you may put target instant or sorcery card from your graveyard on top of your library.

Rootwater Commando

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)

Inertia Bubble

Enchant artifact
Enchanted artifact doesn't untap during its controller's untap step.

Opal Palace

{T}: Add {C}.
{1}, {T}: Add one mana of any color in your commander's color identity. If you spend this mana to cast your commander, it enters with a number of additional +1/+1 counters on it equal to the number of times it's been cast from the command zone this game.

Thassa's Devourer

Constellation — Whenever Thassa's Devourer or another enchantment you control enters, target player mills two cards.

Halimar Depths

Halimar Depths enters tapped.
When Halimar Depths enters, look at the top three cards of your library, then put them back in any order.
{T}: Add {U}.

Merfolk Spy

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Merfolk Spy deals combat damage to a player, that player reveals a card at random from their hand.

Inkfathom Divers

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Inkfathom Divers enters, look at the top four cards of your library, then put them back in any order.

Preordain

Scry 2, then draw a card. (To scry 2, look at the top two cards of your library, then put any number of them on the bottom and the rest on top in any order.)

Path of Ancestry

Path of Ancestry enters tapped.
{T}: Add one mana of any color in your commander's color identity. When that mana is spent to cast a creature spell that shares a creature type with your commander, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Divination

Rootwater Mystic

{1}{U}: Look at the top card of target player's library.

Arcane Denial

Counter target spell. Its controller may draw up to two cards at the beginning of the next turn's upkeep.
You draw a card at the beginning of the next turn's upkeep.

Rootwater Hunter

{T}: Rootwater Hunter deals 1 damage to any target.

Spreading Seas

Enchant land
When Spreading Seas enters, draw a card.
Enchanted land is an Island.

Merfolk Observer

When Merfolk Observer enters, look at the top card of target player's library.

Tidal Warrior

{T}: Target land becomes an Island until end of turn.

Disrupt

Counter target instant or sorcery spell unless its controller pays {1}.
Draw a card.

Eel Umbra

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature gets +1/+1.
Umbra armor (If enchanted creature would be destroyed, instead remove all damage from it and destroy this Aura.)

Tideshaper Mystic

{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.

Reef Shaman

{T}: Target land becomes the basic land type of your choice until end of turn.

Convincing Mirage

Enchant land
As Convincing Mirage enters, choose a basic land type.
Enchanted land is the chosen type.

Annul

Counter target artifact or enchantment spell.

Negate

Arctic Merfolk

Kicker—Return a creature you control to its owner's hand. (You may return a creature you control to its owner's hand in addition to any other costs as you cast this spell.)
If Arctic Merfolk was kicked, it enters with a +1/+1 counter on it.

Phantasmal Terrain

Enchant land
As Phantasmal Terrain enters, choose a basic land type.
Enchanted land is the chosen type.

Augur of Bolas

When Augur of Bolas enters, look at the top three cards of your library. You may reveal an instant or sorcery card from among them and put it into your hand. Put the rest on the bottom of your library in any order.

Scroll Thief

Whenever Scroll Thief deals combat damage to a player, draw a card.

Distant Melody

Choose a creature type. Draw a card for each permanent you control of that type.

Aura Finesse

Attach target Aura you control to target creature.
Draw a card.

Command Tower

{T}: Add one mana of any color in your commander's color identity.

Serum Visions

Vodalian Arcanist

{T}: Add {C}. Spend this mana only to cast an instant or sorcery spell.

Streambed Aquitects

{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.

Island

Stonybrook Banneret

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Merfolk spells and Wizard spells you cast cost {1} less to cast.

Terramorphic Expanse

{T}, Sacrifice Terramorphic Expanse: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.

Armguard Familiar

Ward {2} (Whenever this permanent becomes the target of a spell or ability an opponent controls, counter it unless that player pays {2}.)
Equipped creature gets +2/+1 and has ward {2}.
Reconfigure {4} ({4}: Attach to target creature you control; or unattach from a creature. Reconfigure only as a sorcery. While attached, this isn't a creature.)

Arcane Signet

Deeptread Merrow

{U}: Deeptread Merrow gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)

Otherworldly Gaze

Surveil 3. (Look at the top three cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)
Flashback {1}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Bubble Snare

Kicker {2}{U} (You may pay an additional {2}{U} as you cast this spell.)
Enchant creature
When Bubble Snare enters, if it was kicked, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.

Merrow Harbinger

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
When Merrow Harbinger enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.

Mantle of Tides

Equipped creature gets +1/+2.
Whenever you draw your second card each turn, attach Mantle of Tides to target creature you control.
Equip {3} ({3}: Attach to target creature you control. Equip only as a sorcery.)

Harbor Serpent

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Harbor Serpent can't attack unless there are five or more Islands on the battlefield.

Bonds of Quicksilver

Flash (You may cast this spell any time you could cast an instant.)
Enchant creature
Enchanted creature doesn't untap during its controller's untap step.

Coral Commando

Soaring Seacliff

Soaring Seacliff enters tapped.
When Soaring Seacliff enters, target creature gains flying until end of turn.
{T}: Add {U}.

Salvager of Secrets

When Salvager of Secrets enters, return target instant or sorcery card from your graveyard to your hand.

Counsel of the Soratami

Brainstorm

Draw three cards, then put two cards from your hand on top of your library in any order.

Sea's Claim

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

Mordenkainen's Polymorph

Until end of turn, target creature becomes a Dragon with base power and toughness 4/4 and gains flying. (It loses all other creature types.)

Flight of Fancy

Enchant creature
When Flight of Fancy enters, draw two cards.
Enchanted creature has flying.

Umara Entangler

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)

Ghostform

Up to two target creatures can't be blocked this turn.

Flood of Recollection

Return target instant or sorcery card from your graveyard to your hand. Exile Flood of Recollection.

Crystal Grotto

When Crystal Grotto enters, scry 1.
{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Witching Well

When Witching Well enters, scry 2. (Look at the top two cards of your library, then put any number of them on the bottom and the rest on top in any order.)
{3}{U}, Sacrifice Witching Well: Draw two cards.

Courier's Capsule

{1}{U}, {T}, Sacrifice Courier's Capsule: Draw two cards.

Unsummon

Return target creature to its owner's hand.

Skyline Cascade

Skyline Cascade enters tapped.
When Skyline Cascade enters, target creature an opponent controls doesn't untap during its controller's next untap step.
{T}: Add {U}.

Aquitect's Will

Put a flood counter on target land. That land is an Island in addition to its other types for as long as it has a flood counter on it. If you control a Merfolk, draw a card.