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.
{1}{U}
Arctic Merfolk
Creature - Merfolk
Kicker--Return a creature you control to its owner's hand. (You may return a creature you control to its owner's hand in addition to any other costs as you cast this spell.)
If Arctic Merfolk was kicked, it enters the battlefield with a +1/+1 counter on it.
1 /
1
{U}
Benthic Biomancer
Creature - Merfolk Wizard Mutant
{1}{U}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.)
Whenever one or more +1/+1 counters are put on Benthic Biomancer, draw a card, then discard a card.
1 /
1
{U}
Brainstorm
Instant
Draw three cards, then put two cards from your hand on top of your library in any order.
{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}{U}
Counterspell
Instant
Counter target spell.
{U}
Cursecatcher
Creature - Merfolk Wizard
Sacrifice Cursecatcher: Counter target instant or sorcery spell unless its controller pays {1}.
1 /
1
{1}{U}
Daze
Instant
You may return an Island you control to its owner's hand rather than pay this spell's mana cost.
Counter target spell unless its controller pays {1}.
{U}{U}
Familiar's Ruse
Instant
As an additional cost to cast this spell, return a creature you control to its owner's hand.
Counter target spell.
--
Flooded Strand
Land
{T}, Pay 1 life, Sacrifice Flooded Strand: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.
{U}
Force Spike
Instant
Counter target spell unless its controller pays {1}.
{1}{U}{U}
Force of Negation
Instant
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.
{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
{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
{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}
Mana Leak
Instant
Counter target spell unless its controller pays {3}.
{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
{U}{U}
Merfolk Trickster
Creature - Merfolk Wizard
Flash
When Merfolk Trickster enters the battlefield, tap target creature an opponent controls. It loses all abilities until end of turn.
2 /
2
{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
{U}
Mist-Cloaked Herald
Creature - Merfolk Warrior
Mist-Cloaked Herald can't be blocked.
1 /
1
--
Misty Rainforest
Land
{T}, Pay 1 life, Sacrifice Misty Rainforest: Search your library for a Forest or Island card, put it onto the battlefield, then shuffle.
--
Mutavault
Land
{T}: Add {C}.
{1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.
--
Mystic Sanctuary
Land - Island
({T}: Add {U}.)
Mystic Sanctuary enters the battlefield tapped unless you control three or more other Islands.
When Mystic Sanctuary enters the battlefield untapped, you may put target instant or sorcery card from your graveyard on top of your library.
{1}{U}
Negate
Instant
Counter target noncreature spell.
{U}
Opt
Instant
Scry 1.
Draw a card.
--
Polluted Delta
Land
{T}, Pay 1 life, Sacrifice Polluted Delta: Search your library for an Island or Swamp card, put it onto the battlefield, then shuffle.
{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}
Preordain
Sorcery
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.)
--
Prismatic Vista
Land
{T}, Pay 1 life, Sacrifice Prismatic Vista: Search your library for a basic land card, put it onto the battlefield, then shuffle.
--
Scalding Tarn
Land
{T}, Pay 1 life, Sacrifice Scalding Tarn: Search your library for an Island or Mountain card, put it onto the battlefield, then shuffle.
{U}
Serum Visions
Sorcery
Draw a card. Scry 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
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
--
Snow-Covered Island
Basic Snow Land - Island
({T}: Add {U}.)
{U}
Spell Pierce
Instant
Counter target noncreature spell unless its controller pays {2}.
{U}
Spell Snare
Instant
Counter target spell with mana value 2.
{1}{U}
Spreading Seas
Enchantment - Aura
Enchant land
When Spreading Seas enters the battlefield, draw a card.
Enchanted land is an Island.
{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
{U}
Triton Shorestalker
Creature - Merfolk Rogue
Triton Shorestalker can't be blocked.
1 /
1
{1}{U}{U}
True-Name Nemesis
Creature - Merfolk Rogue
As True-Name Nemesis enters the battlefield, choose a player.
True-Name Nemesis has protection from the chosen player.