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.
{4}{U}{U}
Archetype of Imagination
Enchantment Creature - Human Wizard
Creatures you control have flying.
Creatures your opponents control lose flying and can't have or gain flying.
3 /
2
{4}{U}{U}
Back from the Brink
Enchantment
Exile a creature card from your graveyard and pay its mana cost: Create a token that's a copy of that card. Activate only as a sorcery.
{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.
--
Blighted Cataract
Land
{T}: Add {C}.
{5}{U}, {T}, Sacrifice Blighted Cataract: Draw two cards.
{U}
Brainstorm
Instant
Draw three cards, then put two cards from your hand on top of your library in any order.
{6}
Brass Herald
Artifact Creature - Golem
As Brass Herald enters the battlefield, choose a creature type.
When Brass Herald enters the battlefield, reveal the top four cards of your library. Put all creature cards of the chosen type revealed this way into your hand and the rest on the bottom of your library in any order.
Creatures of the chosen type get +1/+1.
2 /
2
{1}{U}{U}
Cackling Counterpart
Instant
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.)
--
Cephalid Coliseum
Land
{T}: Add {U}. Cephalid Coliseum deals 1 damage to you.
Threshold -- {U}, {T}, Sacrifice Cephalid Coliseum: Target player draws three cards, then discards three cards. Activate only if seven or more cards are in your graveyard.
{3}{U}
Clone
Creature - Shapeshifter
You may have Clone enter the battlefield as a copy of any creature on the battlefield.
0 /
0
{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.
{U}{U}
Coralhelm Commander
Creature - Merfolk Soldier
Level up {1} ({1}: Put a level counter on this. Level up only as a sorcery.)
LEVEL 2-3
3/3
Flying
LEVEL 4+
4/4
Flying
Other Merfolk creatures you control get +1/+1.
2 /
2
{U}
Cosi's Trickster
Creature - Merfolk Wizard
Whenever an opponent shuffles their library, you may put a +1/+1 counter on Cosi's Trickster.
1 /
1
{4}{U}{U}
Crush of Tentacles
Sorcery
Surge {3}{U}{U} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Return all nonland permanents to their owners' hands. If this spell's surge cost was paid, create an 8/8 blue Octopus creature token.
{1}{U}
Cyclonic Rift
Instant
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.")
{5}{U}
Deepchannel Mentor
Creature - Merfolk Rogue
Blue creatures you control can't be blocked.
2 /
2
{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.)
{3}{U}{U}
Devastation Tide
Sorcery
Return all nonland permanents to their owners' hands.
Miracle {1}{U} (You may cast this card for its miracle cost when you draw it if it's the first card you drew this turn.)
{3}{U}
Distant Melody
Sorcery
Choose a creature type. Draw a card for each permanent you control of that type.
{4}
Door of Destinies
Artifact
As Door of Destinies enters the battlefield, choose a creature type.
Whenever you cast a spell of the chosen type, put a charge counter on Door of Destinies.
Creatures you control of the chosen type get +1/+1 for each charge counter on Door of Destinies.
{T}, Sacrifice Ghost Quarter: Destroy target land. Its controller may search their library for a basic land card, put it onto the battlefield, then shuffle.
{3}{U}
Gigantoplasm
Creature - Shapeshifter
You may have Gigantoplasm enter the battlefield as a copy of any creature on the battlefield, except it has "{X}: This creature has base power and toughness X/X."
0 /
0
--
Halimar Depths
Land
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}.
{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
{4}
Hedron Archive
Artifact
{T}: Add {C}{C}.
{2}, {T}, Sacrifice Hedron Archive: Draw two cards.
{2}{U}
Imprisoned in the Moon
Enchantment - Aura
Enchant creature, land, or planeswalker
Enchanted permanent is a colorless land with "{T}: Add {C}" and loses all other card types and abilities.
{3}{U}{U}{U}
Inundate
Sorcery
Return all nonblue creatures to their owners' hands.
--
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}.)
--
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}.)
{3}
Kefnet's Monument
Legendary Artifact
Blue creature spells you cast cost {1} less to cast.
Whenever you cast a creature spell, target creature an opponent controls doesn't untap during its controller's next untap step.
{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
{1}{U}
Lingering Mirage
Enchantment - Aura
Enchant land
Enchanted land is an Island.
Cycling {2} ({2}, Discard this card: Draw a card.)
{U}{U}
Lord of Atlantis
Creature - Merfolk
Other Merfolk get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)
2 /
2
{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, 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
{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}
Metallic Mimic
Artifact Creature - Shapeshifter
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.
2 /
1
{2}
Mind Stone
Artifact
{T}: Add {C}.
{1}, {T}, Sacrifice Mind Stone: Draw a card.
{2}
Mirror of the Forebears
Artifact
As Mirror of the Forebears enters the battlefield, choose a creature type.
{1}: Until end of turn, Mirror of the Forebears becomes a copy of target creature you control of the chosen type, except it's an artifact in addition to its other types.
--
Moonring Island
Land - Island
({T}: Add {U}.)
Moonring Island enters the battlefield tapped.
{U}, {T}: Look at the top card of target player's library. Activate only if you control two or more blue permanents.
--
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.
--
Mystifying Maze
Land
{T}: Add {C}.
{4}, {T}: Exile target attacking creature an opponent controls. At the beginning of the next end step, return it to the battlefield tapped under its owner's control.
--
Opal Palace
Land
{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 the battlefield 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.
--
Path of Ancestry
Land
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.)
{U}{U}
Phantasmal Terrain
Enchantment - Aura
Enchant land
As Phantasmal Terrain enters the battlefield, choose a basic land type.
Enchanted land is the chosen type.
{U}
Ponder
Sorcery
Look at the top three cards of your library, then put them back in any order. You may shuffle.
Draw a card.
{U}
Pongify
Instant
Destroy target creature. It can't be regenerated. Its controller creates a 3/3 green Ape creature token.
{U}
Portent
Sorcery
Look at the top three cards of target player's library, then put them back in any order. You may have that player shuffle.
Draw a card at the beginning of the next turn's upkeep.
{5}{U}{U}
Quicksilver Gargantuan
Creature - Shapeshifter
You may have Quicksilver Gargantuan enter the battlefield as a copy of any creature on the battlefield, except it's 7/7.
7 /
7
{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.
{1}{U}
Reality Shift
Instant
Exile target creature. Its controller manifests the top card of their library. (That player puts the top card of their library onto the battlefield face down as a 2/2 creature. If it's a creature card, it can be turned face up any time for its mana cost.)
--
Reliquary Tower
Land
You have no maximum hand size.
{T}: Add {C}.
{2}{U}{U}
Rite of Replication
Sorcery
Kicker {5} (You may pay an additional {5} as you cast this spell.)
Create a token that's a copy of target creature. If this spell was kicked, create five of those tokens instead.
{1}{U}
River Sneak
Creature - Merfolk Warrior
River Sneak can't be blocked.
Whenever another Merfolk enters the battlefield under your control, River Sneak gets +1/+1 until end of turn.
1 /
1
{4}{U}{U}
River's Rebuke
Sorcery
Return all nonland permanents target player controls to their owner's hand.
--
Rogue's Passage
Land
{T}: Add {C}.
{4}, {T}: Target creature can't be blocked this turn.
{1}{U}
Rootwater Thief
Creature - Merfolk Rogue
{U}: Rootwater Thief gains flying until end of turn.
Whenever Rootwater Thief deals combat damage to a player, you may pay {2}. If you do, search that player's library for a card and exile it, then the player shuffles.
1 /
2
{2}
Sapphire Medallion
Artifact
Blue spells you cast cost {1} less to cast.
{5}{U}{U}
Scourge of Fleets
Creature - Kraken
When Scourge of Fleets enters the battlefield, return each creature your opponents control with toughness X or less to its owner's hand, where X is the number of Islands you control.
6 /
6
{2}{U}
Scroll Thief
Creature - Merfolk Rogue
Whenever Scroll Thief deals combat damage to a player, draw a card.
1 /
3
{U}
Sea's Claim
Enchantment - Aura
Enchant land (Target a land as you cast this. This card enters the battlefield attached to that land.)
Enchanted land is an Island.
{1}{U}{U}
Seasinger
Creature - Merfolk
When you control no Islands, sacrifice Seasinger.
You may choose not to untap Seasinger during your untap step.
{T}: Gain control of target creature whose controller controls an Island for as long as you control Seasinger and Seasinger remains tapped.
0 /
1
{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}{U}
Silvergill Adept
Creature - Merfolk Wizard
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.
2 /
1
--
Skyline Cascade
Land
Skyline Cascade enters the battlefield tapped.
When Skyline Cascade enters the battlefield, target creature an opponent controls doesn't untap during its controller's next untap step.
{T}: Add {U}.
{1}
Sol Ring
Artifact
{T}: Add {C}{C}.
{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
{5}{U}{U}{U}
Stormtide Leviathan
Creature - Leviathan
Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
All lands are Islands in addition to their other types.
Creatures without flying or islandwalk can't attack.
8 /
8
{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
{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
{2}{U}{U}
Tempest Caller
Creature - Merfolk Wizard
When Tempest Caller enters, tap all creatures target opponent controls.
2 /
3
{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
{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
{2}
Thought Vessel
Artifact
You have no maximum hand size.
{T}: Add {C}.
{5}
Vanquisher's Banner
Artifact
As Vanquisher's Banner enters the battlefield, choose a creature type.
Creatures you control of the chosen type get +1/+1.
Whenever you cast a creature spell of the chosen type, draw a card.