Have

Curiosity

Enchant creature
Whenever enchanted creature deals damage to an opponent, you may draw a card.

Chasm Skulker

Whenever you draw a card, put a +1/+1 counter on Chasm Skulker.
When Chasm Skulker dies, create X 1/1 blue Squid creature tokens with islandwalk, where X is the number of +1/+1 counters on Chasm Skulker. (They can't be blocked as long as defending player controls an Island.)

Ice Out

Bargain (You may sacrifice an artifact, enchantment, or token as you cast this spell.)
This spell costs {1} less to cast if it's bargained.
Counter target spell.

Getting

Mystical Tutor

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

Maybe

Solve the Equation

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

Orvar, the All-Form

Changeling
Whenever you cast an instant or sorcery spell, if it targets one or more other permanents you control, create a token that's a copy of one of those permanents.
When a spell or ability an opponent controls causes you to discard this card, create a token that's a copy of target permanent.

Counterspell

Compulsion

{1}{U}, Discard a card: Draw a card.
{1}{U}, Sacrifice Compulsion: Draw a card.

Talrand, Sky Summoner

Whenever you cast an instant or sorcery spell, create a 2/2 blue Drake creature token with flying.

Mitotic Manipulation

Look at the top seven cards of your library. You may put one of those cards onto the battlefield if it has the same name as a permanent. Put the rest on the bottom of your library in any order.

Mist Dancer

Flying
Other Merfolk you control get +1/+0 and have flying.
Encore {5}{U}{U} ({5}{U}{U}, Exile this card from your graveyard: For each opponent, create a token copy that attacks that opponent this turn if able. They gain haste. Sacrifice them at the beginning of the next end step. Activate only as a sorcery.)

Freeze in Place

Tap target creature an opponent controls and put three stun counters on it. Scry 2. (If a permanent with a stun counter would become untapped, remove one from it instead.)

Aquatic Incursion

When Aquatic Incursion enters, create two 1/1 blue Merfolk creature tokens with hexproof. (They can't be the targets of spells or abilities your opponents control.)
{3}{U}: Target Merfolk can't be blocked this turn.

Jace, Unraveler of Secrets

+1: Scry 1, then draw a card.
−2: Return target creature to its owner's hand.
−8: You get an emblem with "Whenever an opponent casts their first spell each turn, counter that spell."

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.

Noble Benefactor

When Noble Benefactor dies, each player may search their library for a card and put that card into their hand. Then each player who searched their library this way shuffles.

Seafloor Oracle

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

Island

Memory Drain

Deny the Divine

Counter target creature or enchantment spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Jace, Arcane Strategist

Whenever you draw your second card each turn, put a +1/+1 counter on target creature you control.
+1: Draw a card.
−7: Creatures you control can't be blocked this turn.

Essence Scatter

Propaganda

Creatures can't attack you unless their controller pays {2} for each creature they control that's attacking you.

Teferi, Temporal Archmage

+1: Look at the top two cards of your library. Put one of them into your hand and the other on the bottom of your library.
−1: Untap up to four target permanents.
−10: You get an emblem with "You may activate loyalty abilities of planeswalkers you control on any player's turn any time you could cast an instant."
Teferi, Temporal Archmage can be your commander.

Spell Blast

Counter target spell with mana value X. (For example, if that spell's mana cost is {3}{U}{U}, X is 5.)

Unsummon

Return target creature to its owner's hand.

Contentious Plan

Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)
Draw a card.

Impede Momentum

Tap target creature and put three stun counters on it. (If a permanent with a stun counter would become untapped, remove one from it instead.)
Scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Ichormoon Gauntlet

Planeswalkers you control have "[0]: Proliferate" and "[−12]: Take an extra turn after this one."
Whenever you cast a noncreature spell, choose a counter on target permanent. Put an additional counter of that kind on that permanent.

Misleading Motes

Target creature's owner puts it on their choice of the top or bottom of their library.

Merfolk Sovereign

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.

Gilded Lotus

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.

Volshe Tideturner

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

Commandeer

You may exile two blue cards from your hand rather than pay this spell's mana cost.
Gain control of target noncreature spell. You may choose new targets for it. (If that spell is an artifact, enchantment, or planeswalker, the permanent enters under your control.)

Ichthyomorphosis

Enchant creature
Enchanted creature loses all abilities and is a blue Fish with base power and toughness 0/1.

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.

High Tide

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

Ertai's Scorn

This spell costs {U} less to cast if an opponent cast two or more spells this turn.
Counter target spell.

Cackling Counterpart

Create a token that's a copy of target creature you control.
Flashback {5}{U}{U} (You may cast this card from your graveyard for its flashback cost. Then exile it.)

Lord of Atlantis

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

Jace, Ingenious Mind-Mage

+1: Draw a card.
+1: Untap all creatures you control.
−9: Gain control of up to three target creatures.

Seahunter

{3}, {T}: Search your library for a Merfolk permanent card, put it onto the battlefield, then shuffle.

Access Denied

Counter target spell. Create X 1/1 colorless Thopter artifact creature tokens with flying, where X is that spell's mana value.

Callaphe, Beloved of the Sea

Callaphe's power is equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
Creatures and enchantments you control have "Spells your opponents cast that target this permanent cost {1} more to cast."

Remove Soul

Merrow Reejerey

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

One with the Stars

Enchant creature or enchantment
Enchanted permanent is an enchantment and loses all other card types. (It still has its abilities, but it's no longer a creature.)

Hidden Cataract

Hidden Cataract enters tapped.
{T}: Add {U}.
{4}{U}, {T}, Sacrifice Hidden Cataract: Discover 4. Activate only as a sorcery. (Exile cards from the top of your library until you exile a nonland card with mana value 4 or less. Cast it without paying its mana cost or put it into your hand. Put the rest on the bottom in a random order.)

Towering-Wave Mystic

Whenever Towering-Wave Mystic deals damage, target player mills that many cards.

Long-Term Plans

Search your library for a card, then shuffle and put that card third from the top.

Ghost-Lit Drifter

Flying
{2}{U}: Another target creature gains flying until end of turn.
Channel — {X}{U}, Discard Ghost-Lit Drifter: X target creatures gain flying until end of turn.

Nadir Kraken

Whenever you draw a card, you may pay {1}. If you do, put a +1/+1 counter on Nadir Kraken and create a 1/1 blue Tentacle creature token.

Minor Misstep

Counter target spell with mana value 1 or less.

Totally Lost

Put target nonland permanent on top of its owner's library.

Teferi, Timeless Voyager

+1: Draw a card.
−3: Put target creature on top of its owner's library.
−8: Each creature target opponent controls phases out. Until the end of your next turn, they can't phase in. (Treat them and anything attached to them as though they don't exist.)

Tezzeret's Gambit

({U/P} can be paid with either {U} or 2 life.)
Draw two cards, then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Blue Sun's Twilight

Gain control of target creature with mana value X or less. If X is 5 or more, create a token that's a copy of that creature.

Benthicore

When Benthicore enters, create two 1/1 blue Merfolk Wizard creature tokens.
Tap two untapped Merfolk you control: Untap Benthicore. It gains shroud until end of turn. (It can't be the target of spells or abilities.)

Seat of the Synod

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.

Transcendent Message

Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.)
Draw X cards.

Merchant Scroll

Search your library for a blue instant card, reveal that card, put it into your hand, then shuffle.

Brackish Blunder

Return target creature to its owner's hand. If it was tapped, create a Map token. (It's an artifact with "{1}, {T}, Sacrifice this artifact: Target creature you control explores. Activate only as a sorcery.")

Hurl into History

Counter target artifact or creature spell. Discover X, where X is that spell's mana value. (Exile cards from the top of your library until you exile a nonland card with that mana value or less. Cast it without paying its mana cost or put it into your hand. Put the rest on the bottom in a random order.)

Vodalian Hexcatcher

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

Sleep Paralysis

Enchant creature
When Sleep Paralysis enters, tap enchanted creature.
Enchanted creature doesn't untap during its controller's untap step.

Stern Dismissal

Return target creature or enchantment an opponent controls to its owner's hand.

Fading Hope

Return target creature to its owner's hand. If its mana value was 3 or less, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Sol Ring

Sleep of the Dead

Tap target creature. It doesn't untap during its controller's next untap step.
Escape—{2}{U}, Exile three other cards from your graveyard. (You may cast this card from your graveyard for its escape cost.)

Retraced Image

Reveal a card in your hand, then put that card onto the battlefield if it has the same name as a permanent.

Stasis Field

Enchant creature
Enchanted creature has base power and toughness 0/2, has defender, and loses all other abilities.

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.

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.)

Sleight of Hand

Look at the top two cards of your library. Put one of them into your hand and the other on the bottom of your library.

An Offer You Can't Refuse

Counter target noncreature spell. Its controller creates two Treasure tokens. (They're artifacts with "{T}, Sacrifice this token: Add one mana of any color.")

Shipwreck Dowser

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
When this creature enters, return target instant or sorcery card from your graveyard to your hand.

Negate

Personal Tutor

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