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.)

Seafloor Oracle

Whenever a Merfolk you control deals combat damage to a player, draw a card.

Archmage's Charm

Choose one —
• Counter target spell.
• Target player draws two cards.
• Gain control of target nonland permanent with mana value 1 or less.

Fierce Guardianship

If you control a commander, you may cast this spell without paying its mana cost.
Counter target noncreature spell.

Silvergill Adept

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

Lord of Atlantis

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

Mist-Cloaked Herald

Metallic Mimic

As Metallic Mimic enters the battlefield, choose a creature type.
Metallic Mimic is the chosen type in addition to its other types.
Each other creature you control of the chosen type enters the battlefield with an additional +1/+1 counter on it.

Force of Negation

If it's not your turn, you may exile a blue card from your hand rather than pay this spell's mana cost.
Counter target noncreature spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Serum Visions

Kopala, Warden of Waves

Spells your opponents cast that target a Merfolk you control cost {2} more to cast.
Abilities your opponents activate that target a Merfolk you control cost {2} more to activate.

Flare of Denial

You may sacrifice a nontoken blue creature rather than pay this spell's mana cost.
Counter target spell.

Flow of Knowledge

Draw a card for each Island you control, then discard two cards.

Deeproot Waters

Whenever you cast a Merfolk spell, create a 1/1 blue Merfolk creature token with hexproof. (A creature with hexproof can't be the target of spells or abilities your opponents control.)

Cryptic Command

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

Solve the Equation

Search your library for an instant or sorcery card, reveal it, put it into your hand, then shuffle.

Cursecatcher

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

Rapid Hybridization

Destroy target creature. It can't be regenerated. That creature's controller creates a 3/3 green Frog Lizard creature token.

Tideshaper Mystic

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

Kindred Discovery

As Kindred Discovery enters the battlefield, choose a creature type.
Whenever a creature you control of the chosen type enters the battlefield or attacks, draw a card.

Sea Gate, Reborn

As Sea Gate, Reborn enters the battlefield, you may pay 3 life. If you don't, it enters the battlefield tapped.
{T}: Add {U}.

Sea Gate Restoration

Draw cards equal to the number of cards in your hand plus one. You have no maximum hand size for the rest of the game.

Sea Gate Restoration // Sea Gate, Reborn

Soporific Springs

As Soporific Springs enters the battlefield, you may pay 3 life. If you don't, it enters the battlefield tapped.
{T}: Add {U}.

Sink into Stupor

Return target spell or nonland permanent an opponent controls to its owner's hand.

Sink into Stupor // Soporific Springs

Force of Will

You may pay 1 life and exile a blue card from your hand rather than pay this spell's mana cost.
Counter target spell.

Deeproot Pilgrimage

Whenever one or more nontoken Merfolk you control become tapped, create a 1/1 blue Merfolk creature token with hexproof.

Sol Ring

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.

Lullmage Mentor

Whenever a spell or ability you control counters a spell, you may create a 1/1 blue Merfolk creature token.
Tap seven untapped Merfolk you control: Counter target spell.

Raise the Palisade

Choose a creature type. Return all creatures that aren't of the chosen type to their owners' hands.

Arcane Signet

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

Tide Shaper

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

Bident of Thassa

Whenever a creature you control deals combat damage to a player, you may draw a card.
{1}{U}, {T}: Creatures your opponents control attack this turn if able.

Mistway Spy

Flying
Disguise {1}{U} (You may cast this card face down for {3} as a 2/2 creature with ward {2}. Turn it face up any time for its disguise cost.)
When Mistway Spy is turned face up, until end of turn, whenever a creature you control deals combat damage to a player, investigate.

Thieving Skydiver

Kicker {X}. X can't be 0. (You may pay an additional {X} as you cast this spell.)
Flying
When Thieving Skydiver enters the battlefield, if it was kicked, gain control of target artifact with mana value X or less. If that artifact is an Equipment, attach it to Thieving Skydiver.

Triton Shorestalker

Vedalken Shackles

You may choose not to untap Vedalken Shackles during your untap step.
{2}, {T}: Gain control of target creature with power less than or equal to the number of Islands you control for as long as Vedalken Shackles remains tapped.

Strix Serenade

Counter target artifact, creature, or planeswalker spell. Its controller creates a 2/2 blue Bird creature token with flying.

Tishana's Tidebinder

Flash
When Tishana's Tidebinder enters the battlefield, counter up to one target activated or triggered ability. If an ability of an artifact, creature, or planeswalker is countered this way, that permanent loses all abilities for as long as Tishana's Tidebinder remains on the battlefield. (Mana abilities can't be targeted.)

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.

Mana Drain

Counter target spell. At the beginning of your next main phase, add an amount of {C} equal to that spell's mana value.

Mistcaller

Sacrifice Mistcaller: Until end of turn, if a nontoken creature would enter the battlefield and it wasn't cast, exile it instead.

Cosi's Trickster

Whenever an opponent shuffles their library, you may put a +1/+1 counter on Cosi's Trickster.

Merfolk Trickster

Flash
When Merfolk Trickster enters the battlefield, tap target creature an opponent controls. It loses all abilities until end of turn.

Swan Song

Counter target enchantment, instant, or sorcery spell. Its controller creates a 2/2 blue Bird creature token with flying.

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.)

Coat of Arms

Each creature gets +1/+1 for each other creature on the battlefield that shares at least one creature type with it. (For example, if two Goblin Warriors and a Goblin Shaman are on the battlefield, each gets +2/+2.)

Merrow Reejerey

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

Dispel

Vodalian Hexcatcher

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

Ponder

Look at the top three cards of your library, then put them back in any order. You may shuffle.
Draw a card.

Rishadan Dockhand

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

Pongify

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

Harbinger of the Seas

Merfolk Windrobber

Flying
Whenever Merfolk Windrobber deals combat damage to a player, that player mills a card. (They put the top card of their library into their graveyard.)
Sacrifice Merfolk Windrobber: Draw a card. Activate only if an opponent has eight or more cards in their graveyard.

Mystical Tutor

Search your library for an instant or sorcery card, reveal it, then shuffle and put that card on top.

Emperor Mihail II

You may look at the top card of your library any time.
You may cast Merfolk spells from the top of your library.
Whenever you cast a Merfolk spell, you may pay {1}. If you do, create a 1/1 blue Merfolk creature token.

Cyclonic Rift

Return target nonland permanent you don't control to its owner's hand.
Overload {6}{U} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

High Tide

Until end of turn, whenever a player taps an Island for mana, that player adds an additional {U}.