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.
{2}
Arcane Signet
Artifact
{T}: Add one mana of any color in your commander's color identity.
--
Azorius Guildgate
Land - Gate
Azorius Guildgate enters the battlefield tapped.
{T}: Add {W} or {U}.
{2}
Azorius Signet
Artifact
{1}, {T}: Add {W}{U}.
{2}{U}{U}
Bident of Thassa
Legendary Enchantment Artifact
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.
{3}
Commander's Sphere
Artifact
{T}: Add one mana of any color in your commander's color identity.
Sacrifice Commander's Sphere: Draw a card.
--
Darksteel Citadel
Artifact Land
Indestructible
{T}: Add {C}.
{3}
Darksteel Ingot
Artifact
Indestructible
{T}: Add one mana of any color.
{1}{W}
Dawn Charm
Instant
Choose one --
• Prevent all combat damage that would be dealt this turn.
• Regenerate target creature.
• Counter target spell that targets you.
{1}{U}
Deeproot Pilgrimage
Enchantment
Whenever one or more nontoken Merfolk you control become tapped, create a 1/1 blue Merfolk creature token with hexproof.
{2}{U}
Deeproot Waters
Enchantment
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.)
{2}{U}
Drowner of Secrets
Creature - Merfolk Wizard
Tap an untapped Merfolk you control: Target player mills a card.
1 /
3
{1}
Elixir of Immortality
Artifact
{2}, {T}: You gain 5 life. Shuffle Elixir of Immortality and your graveyard into their owner's library.
{1}{U}{U}
Emperor Mihail II
Legendary Creature - Merfolk Noble
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.
3 /
3
--
Evolving Wilds
Land
{T}, Sacrifice Evolving Wilds: Search your library for a basic land card, put it onto the battlefield tapped, then shuffle.
{3}{U}
Fallowsage
Creature - Merfolk Wizard
Whenever Fallowsage becomes tapped, you may draw a card.
2 /
2
--
Glacial Fortress
Land
Glacial Fortress enters the battlefield tapped unless you control a Plains or an Island.
{T}: Add {W} or {U}.
{U}{U}
Harbinger of the Tides
Creature - Merfolk Wizard
You may cast Harbinger of the Tides as though it had flash if you pay {2} more to cast it. (You may cast it any time you could cast an instant.)
When Harbinger of the Tides enters the battlefield, you may return target tapped creature an opponent controls to its owner's hand.
2 /
2
{2}{W}
Harpoon Sniper
Creature - Merfolk Archer
{W}, {T}: Harpoon Sniper deals X damage to target attacking or blocking creature, where X is the number of Merfolk you control.
2 /
2
{4}
Hedron Archive
Artifact
{T}: Add {C}{C}.
{2}, {T}, Sacrifice Hedron Archive: Draw two cards.
{3}{U}
Herald of Secret Streams
Creature - Merfolk Warrior
Creatures you control with +1/+1 counters on them can't be blocked.
2 /
3
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
--
Island
Basic Land - Island
({T}: Add {U}.)
{1}{W}
Judge of Currents
Creature - Merfolk Wizard
Whenever a Merfolk you control becomes tapped, you may gain 1 life.
1 /
1
{3}{U}{U}
Kindred Discovery
Enchantment
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.
{1}{U}{U}
Kopala, Warden of Waves
Legendary Creature - Merfolk Wizard
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.
2 /
2
{2}{W}{W}
Leyline of Sanctity
Enchantment
If Leyline of Sanctity is in your opening hand, you may begin the game with it on the battlefield.
You have hexproof. (You can't be the target of spells or abilities your opponents control.)
{1}{U}{U}
Lullmage Mentor
Creature - Merfolk Wizard
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.
2 /
2
{3}{U}
Master of Waves
Creature - Merfolk Wizard
Protection from red
Elemental creatures you control get +1/+1.
When Master of Waves enters the battlefield, create a number of 1/0 blue Elemental creature tokens equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
2 /
1
{U}{U}
Master of the Pearl Trident
Creature - Merfolk
Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)
2 /
2
--
Meandering River
Land
Meandering River enters the battlefield tapped.
{T}: Add {W} or {U}.
{1}{U}{U}
Merfolk Sovereign
Creature - Merfolk Noble
Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.
2 /
2
{1}{U}
Merrow Commerce
Tribal Enchantment - Merfolk
At the beginning of your end step, untap all Merfolk you control.
{3}{U}
Merrow Harbinger
Creature - Merfolk Wizard
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.
2 /
3
{2}{U}
Merrow Reejerey
Creature - Merfolk Soldier
Other Merfolk creatures you control get +1/+1.
Whenever you cast a Merfolk spell, you may tap or untap target permanent.
2 /
2
{2}
Mind Stone
Artifact
{T}: Add {C}.
{1}, {T}, Sacrifice Mind Stone: Draw a card.
{4}{U}
Mist Dancer
Creature - Merfolk Wizard
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.)
3 /
3
--
Myriad Landscape
Land
Myriad Landscape enters the battlefield tapped.
{T}: Add {C}.
{2}, {T}, Sacrifice Myriad Landscape: Search your library for up to two basic land cards that share a land type, put them onto the battlefield tapped, then shuffle.
{3}{W}{U}
Noyan Dar, Roil Shaper
Legendary Creature - Merfolk Ally
Whenever you cast an instant or sorcery spell, you may put three +1/+1 counters on target land you control. If you do, that land becomes a 0/0 Elemental creature with haste that's still a land.
4 /
4
{2}{U}{U}
Opposition
Enchantment
Tap an untapped creature you control: Tap target artifact, creature, or land.
{2}{W}
Pariah
Enchantment - Aura
Enchant creature
All damage that would be dealt to you is dealt to enchanted creature instead.
{5}
Pariah's Shield
Artifact - Equipment
All damage that would be dealt to you is dealt to equipped creature instead.
Equip {3}
{W}
Path to Exile
Instant
Exile target creature. Its controller may search their library for a basic land card, put that card onto the battlefield tapped, then shuffle.
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
--
Plains
Basic Land - Plains
({T}: Add {W}.)
{2}
Planar Atlas
Artifact
Planar Atlas enters the battlefield tapped.
When Planar Atlas enters the battlefield, you may look at the top four cards of your library. If you do, reveal up to one land card from among them, then put that card on top of your library and the rest on the bottom in a random order.
{T}: Add {C}.
{U}
Pongify
Instant
Destroy target creature. It can't be regenerated. Its controller creates a 3/3 green Ape creature token.
{W/U}{W/U}
Puresight Merrow
Creature - Merfolk Wizard
{W/U}, {Q}: Look at the top card of your library. You may exile that card. ({Q} is the untap symbol.)
2 /
2
{U}
Rapid Hybridization
Instant
Destroy target creature. It can't be regenerated. That creature's controller creates a 3/3 green Frog Lizard creature token.
{2}{U}{U}
Seafloor Oracle
Creature - Merfolk Wizard
Whenever a Merfolk you control deals combat damage to a player, draw a card.
2 /
3
{1}{U}{U}
Sigil Tracer
Creature - Merfolk Wizard
{1}{U}, Tap two untapped Wizards you control: Copy target instant or sorcery spell. You may choose new targets for the copy.
2 /
2
{1}
Sol Ring
Artifact
{T}: Add {C}{C}.
{1}{W}{W}
Spear of Heliod
Legendary Enchantment Artifact
Creatures you control get +1/+1.
{1}{W}{W}, {T}: Destroy target creature that dealt damage to you this turn.
{2}{W/U}
Steel of the Godhead
Enchantment - Aura
Enchant creature
As long as enchanted creature is white, it gets +1/+1 and has lifelink. (Damage dealt by the creature also causes its controller to gain that much life.)
As long as enchanted creature is blue, it gets +1/+1 and can't be blocked.
{4}{U}{U}
Stolen Identity
Sorcery
Create a token that's a copy of target artifact or creature.
Cipher (Then you may exile this spell card encoded on a creature you control. Whenever that creature deals combat damage to a player, its controller may cast a copy of the encoded card without paying its mana cost.)
{1}{U}
Stonybrook Banneret
Creature - Merfolk Wizard
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.
1 /
1
{2}{W}
Stonybrook Schoolmaster
Creature - Merfolk Wizard
Whenever Stonybrook Schoolmaster becomes tapped, you may create a 1/1 blue Merfolk Wizard creature token.
1 /
2
{3}{W}
Summon the School
Tribal Sorcery - Merfolk
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.
{2}{U}{U}
Surgespanner
Creature - Merfolk Wizard
Whenever Surgespanner becomes tapped, you may pay {1}{U}. If you do, return target permanent to its owner's hand.
2 /
2
{1}{U}{U}
Svyelun of Sea and Sky
Legendary Creature - Merfolk God
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}.)
3 /
4
{W}
Swords to Plowshares
Instant
Exile target creature. Its controller gains life equal to its power.
{W}{U}
Sygg, River Guide
Legendary Creature - Merfolk Wizard
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
{1}{W}: Target Merfolk you control gains protection from the color of your choice until end of turn.
2 /
2
{2}{U}{U}
Talrand, Sky Summoner
Legendary Creature - Merfolk Wizard
Whenever you cast an instant or sorcery spell, create a 2/2 blue Drake creature token with flying.
2 /
2
{1}{U}{U}
Thada Adel, Acquisitor
Legendary Creature - Merfolk Rogue
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Thada Adel, Acquisitor 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.
2 /
2
{3}{U}
Thassa, Deep-Dwelling
Legendary Enchantment Creature - God
Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature.
At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control.
{3}{U}: Tap another target creature.
6 /
5
{2}{U}
Thassa, God of the Sea
Legendary Enchantment Creature - God
Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)
At the beginning of your upkeep, scry 1.
{1}{U}: Target creature you control can't be blocked this turn.
5 /
5
{1}{U}
Thieving Skydiver
Creature - Merfolk Rogue
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.
2 /
1
{2}
Thought Vessel
Artifact
You have no maximum hand size.
{T}: Add {C}.
{U}{U}
Tidebinder Mage
Creature - Merfolk Wizard
When Tidebinder Mage enters the battlefield, tap target red or green creature an opponent controls. That creature doesn't untap during its controller's untap step for as long as you control Tidebinder Mage.
2 /
2
{2}{U}
Tishana's Tidebinder
Creature - Merfolk Wizard
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.)
3 /
2
--
Tranquil Cove
Land
Tranquil Cove enters the battlefield tapped.
When Tranquil Cove enters the battlefield, you gain 1 life.
{T}: Add {W} or {U}.
{3}{W}
Veteran of the Depths
Creature - Merfolk Soldier
Whenever Veteran of the Depths becomes tapped, you may put a +1/+1 counter on it.
2 /
2
{1}{U}
Vodalian Hexcatcher
Creature - Merfolk Wizard
Flash
Other Merfolk you control get +1/+1.
Sacrifice a Merfolk: Counter target noncreature spell unless its controller pays {1}.
1 /
1
{2}{W}{U}
Vodalian Wave-Knight
Creature - Merfolk Knight
Whenever you draw a card, put a +1/+1 counter on each other Merfolk and/or Knight you control.
3 /
3
{2}{U}
Wake Thrasher
Creature - Merfolk Soldier
Whenever a permanent you control becomes untapped, Wake Thrasher gets +1/+1 until end of turn.
1 /
1
--
Wanderwine Hub
Land
As Wanderwine Hub enters the battlefield, you may reveal a Merfolk card from your hand. If you don't, Wanderwine Hub enters the battlefield tapped.
{T}: Add {W} or {U}.
{1}
Wayfarer's Bauble
Artifact
{2}, {T}, Sacrifice Wayfarer's Bauble: Search your library for a basic land card, put that card onto the battlefield tapped, then shuffle.
{4}{W}
Wellgabber Apothecary
Creature - Merfolk Cleric
{1}{W}: Prevent all damage that would be dealt to target tapped Merfolk or Kithkin creature this turn.