Svyelun of Sea and Sky

Svyelun of Sea and Sky 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}.)

Honor-Worn Shaku

{T}: Add {C}.
Tap an untapped legendary permanent you control: Untap Honor-Worn Shaku.

Mine Excavation

Return target artifact or enchantment card from a graveyard to its owner's hand.
Conspire (As you cast this spell, you may tap two untapped creatures you control that share a color with it. When you do, copy it and you may choose a new target for the copy.)

Candlekeep Sage

Commander creatures you own have "When this creature enters or leaves the battlefield, draw a card."

Blessed Alliance

Escalate {2} (Pay this cost for each mode chosen beyond the first.)
Choose one or more —
• Target player gains 4 life.
• Untap up to two target creatures.
• Target opponent sacrifices an attacking creature.

Breaking Wave

You may cast Breaking Wave as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
Simultaneously untap all tapped creatures and tap all untapped creatures.

Sea Scryer

Lulu, Loyal Hollyphant

Flying
At the beginning of your end step, if a permanent you controlled left the battlefield this turn, put a +1/+1 counter on each tapped creature you control, then untap them.
Choose a Background (You can have a Background as a second commander.)

Angel's Trumpet

All creatures have vigilance.
At the beginning of each player's end step, tap all untapped creatures that player controls that didn't attack this turn. Angel's Trumpet deals damage to the player equal to the number of creatures tapped this way.

Drowner of Secrets

Tap an untapped Merfolk you control: Target player mills 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.)

Sentinel of the Pearl Trident

Flash
When Sentinel of the Pearl Trident enters the battlefield, you may exile target historic permanent you control. If you do, return that card to the battlefield under its owner's control at the beginning of the next end step. (Artifacts, legendaries, and Sagas are historic.)

To Arms!

Untap all creatures you control.
Draw a card.

Seahunter

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

Devout Invocation

Tap any number of untapped creatures you control. Create a 4/4 white Angel creature token with flying for each creature tapped this way.

White Plume Adventurer

When White Plume Adventurer enters the battlefield, you take the initiative.
At the beginning of each opponent's upkeep, untap a creature you control. If you've completed a dungeon, untap all creatures you control instead.

Faces of the Past

Whenever a creature dies, tap all untapped creatures that share a creature type with it or untap all tapped creatures that share a creature type with it.

Ardenvale Fealty

Create a 2/2 white Knight creature token with vigilance. (Then exile this card. You may cast the enchantment later from exile.)

Virtue of Loyalty

At the beginning of your end step, put a +1/+1 counter on each creature you control. Untap those creatures.

Virtue of Loyalty // Ardenvale Fealty

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.

Everything Comes to Dust

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.)
Exile all creatures except those that share a creature type with a creature that convoked this spell, all artifacts, and all enchantments.

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.

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.

Village Bell-Ringer

Flash (You may cast this spell any time you could cast an instant.)
When Village Bell-Ringer enters the battlefield, untap all creatures you control.

Tamiyo, the Moon Sage

+1: Tap target permanent. It doesn't untap during its controller's next untap step.
−2: Draw a card for each tapped creature target player controls.
−8: You get an emblem with "You have no maximum hand size" and "Whenever a card is put into your graveyard from anywhere, you may return it to your hand."

Merrow Reejerey

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

Blinding Beam

Choose one —
• Tap two target creatures.
• Creatures don't untap during target player's next untap step.
Entwine {1} (Choose both if you pay the entwine cost.)

Benthicore

When Benthicore enters the battlefield, 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.)

Clever Concealment

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.)
Any number of target nonland permanents you control phase out. (Treat them and anything attached to them as though they don't exist until your next turn.)

Drumbellower

Flying
Untap all creatures you control during each other player's untap step.

Sky Hussar

Flying
When Sky Hussar enters the battlefield, untap all creatures you control.
Forecast — Tap two untapped white and/or blue creatures you control, Reveal Sky Hussar from your hand: Draw a card. (Activate only during your upkeep and only once each turn.)

Puresight Merrow

{W/U}, {Q}: Look at the top card of your library. You may exile that card. ({Q} is the untap symbol.)

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.

Maze of Ith

{T}: Untap target attacking creature. Prevent all combat damage that would be dealt to and dealt by that creature this turn.

Reconnaissance

{0}: Remove target attacking creature you control from combat and untap it.

Wanderwine Prophets

Champion a Merfolk (When this enters the battlefield, sacrifice it unless you exile another Merfolk you control. When this leaves the battlefield, that card returns to the battlefield.)
Whenever Wanderwine Prophets deals combat damage to a player, you may sacrifice a Merfolk. If you do, take an extra turn after this one.

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.)

Judge of Currents

Whenever a Merfolk you control becomes tapped, you may gain 1 life.

Sigil Tracer

{1}{U}, Tap two untapped Wizards you control: Copy target instant or sorcery spell. You may choose new targets for the copy.

Grand Architect

Other blue creatures you control get +1/+1.
{U}: Target artifact creature becomes blue until end of turn.
Tap an untapped blue creature you control: Add {C}{C}. Spend this mana only to cast artifact spells or activate abilities of artifacts.

Grimoire Thief

Whenever Grimoire Thief becomes tapped, exile the top three cards of target opponent's library face down.
You may look at cards exiled with Grimoire Thief.
{U}, Sacrifice Grimoire Thief: Turn all cards exiled with Grimoire Thief face up. Counter all spells with those names.

Barracks of the Thousand

(Transforms from Thousand Moons Smithy.)
{T}: Add {W}.
Whenever you cast an artifact or creature spell using mana produced by Barracks of the Thousand, create a white Gnome Soldier artifact creature token with "This creature's power and toughness are each equal to the number of artifacts and/or creatures you control."

Thousand Moons Smithy

When Thousand Moons Smithy enters the battlefield, create a white Gnome Soldier artifact creature token with "This creature's power and toughness are each equal to the number of artifacts and/or creatures you control."
At the beginning of your precombat main phase, you may tap five untapped artifacts and/or creatures you control. If you do, transform Thousand Moons Smithy.

Thousand Moons Smithy // Barracks of the Thousand

Summon the School

Create two 1/1 blue Merfolk Wizard creature tokens.
Tap four untapped Merfolk you control: Return Summon the School from your graveyard to your hand.

Fallowsage

Whenever Fallowsage becomes tapped, you may draw a card.

Jace, Ingenious Mind-Mage

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

Halo Fountain

{W}, {T}, Untap a tapped creature you control: Create a 1/1 green and white Citizen creature token.
{W}{W}, {T}, Untap two tapped creatures you control: Draw a card.
{W}{W}{W}{W}{W}, {T}, Untap fifteen tapped creatures you control: You win the game.

Energy Arc

Untap any number of target creatures. Prevent all combat damage that would be dealt to and dealt by those creatures this turn.

Deeproot Pilgrimage

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

Caetus, Sea Tyrant of Segovia

Noncreature spells you cast have convoke. (Your creatures can help cast those spells. Each creature you tap while casting a noncreature spell pays for {1} or one mana of that creature's color.)
At the beginning of your end step, untap up to four target creatures.

Invasion of Segovia

(As a Siege enters, choose an opponent to protect it. You and others can attack it. When it's defeated, exile it, then cast it transformed.)
When Invasion of Segovia enters the battlefield, create two 1/1 blue Kraken creature tokens with trample.

Invasion of Segovia // Caetus, Sea Tyrant of Segovia

Stonybrook Schoolmaster

Whenever Stonybrook Schoolmaster becomes tapped, you may create a 1/1 blue Merfolk Wizard creature token.

Vodalian War Machine

Defender (This creature can't attack.)
Tap an untapped Merfolk you control: Vodalian War Machine can attack this turn as though it didn't have defender.
Tap an untapped Merfolk you control: Vodalian War Machine gets +2/+1 until end of turn.
When Vodalian War Machine dies, destroy all Merfolk tapped this turn to pay for its abilities.

Vodalian Wave-Knight

Whenever you draw a card, put a +1/+1 counter on each other Merfolk and/or Knight you control.

Merrow Harbinger

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

The Fifth Doctor

Peaceful Coexistence — At the beginning of your end step, put a +1/+1 counter on each creature you control that didn't attack or enter the battlefield this turn. Untap those creatures.

Merfolk Skyscout

Flying
Whenever Merfolk Skyscout attacks or blocks, untap target permanent.

Teferi, Who Slows the Sunset

+1: Choose up to one target artifact, up to one target creature, and up to one target land. Untap the chosen permanents you control. Tap the chosen permanents you don't control. You gain 2 life.
−2: Look at the top three cards of your library. Put one of them into your hand and the rest on the bottom of your library in any order.
−7: You get an emblem with "Untap all permanents you control during each opponent's untap step" and "You draw a card during each opponent's draw step."

Diversionary Tactics

Tap two untapped creatures you control: Tap target creature.

Riverwise Augur

When Riverwise Augur enters the battlefield, draw three cards, then put two cards from your hand on top of your library in any order.

Cryptic Gateway

Tap two untapped creatures you control: You may put a creature card from your hand that shares a creature type with each creature tapped this way onto the battlefield.

Wake Thrasher

Whenever a permanent you control becomes untapped, Wake Thrasher gets +1/+1 until end of turn.

Merrow Commerce

At the beginning of your end step, untap all Merfolk you control.

Surgespanner

Whenever Surgespanner becomes tapped, you may pay {1}{U}. If you do, return target permanent to its owner's hand.

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.

Vodalian Hexcatcher

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

Wand of the Worldsoul

Wand of the Worldsoul enters the battlefield tapped.
{T}: Add {W}.
{T}: The next spell you cast this turn has convoke.

Ghastly Discovery

Draw two cards, then discard a card.
Conspire (As you cast this spell, you may tap two untapped creatures you control that share a color with it. When you do, copy it.)