Merfolk Trickster

Flash
When Merfolk Trickster enters, tap target creature an opponent controls. It loses all abilities until end of turn.

Surgespanner

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

Deeproot Waters

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.)

Veteran of the Depths

Whenever Veteran of the Depths becomes tapped, you may put a +1/+1 counter on it.

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.

Ink Dissolver

Kinship — At the beginning of your upkeep, you may look at the top card of your library. If it shares a creature type with Ink Dissolver, you may reveal it. If you do, each opponent mills three cards.

Sigil Tracer

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

Cosi's Trickster

Whenever an opponent shuffles their library, you may put a +1/+1 counter on Cosi's Trickster.

Merrow Reejerey

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

Azorius Signet

Heirloom Blade

Equipped creature gets +3/+1.
Whenever equipped creature dies, you may reveal cards from the top of your library until you reveal a creature card that shares a creature type with it. Put that card into your hand and the rest on the bottom of your library in a random order.
Equip {1}

Stonybrook Angler

{1}{U}, {T}: You may tap or untap target creature.

Lost Vale

(Transforms from Dowsing Dagger.)
{T}: Add three mana of any one color.

Dowsing Dagger

When Dowsing Dagger enters, target opponent creates two 0/2 green Plant creature tokens with defender.
Equipped creature gets +2/+1.
Whenever equipped creature deals combat damage to a player, you may transform Dowsing Dagger.
Equip {2}

Dowsing Dagger // Lost Vale

Wake Thrasher

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

Sejiri Merfolk

As long as you control a Plains, Sejiri Merfolk has first strike and lifelink. (Damage dealt by a creature with lifelink also causes its controller to gain that much life.)

Kefnet's Monument

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.

Seafloor Oracle

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

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.

Grafted Exoskeleton

Equipped creature gets +2/+2 and has infect. (It deals damage to creatures in the form of -1/-1 counters and to players in the form of poison counters.)
Whenever Grafted Exoskeleton becomes unattached from a permanent, sacrifice that permanent.
Equip {2}

Stonybrook Schoolmaster

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

Merfolk Sovereign

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

Merfolk Mesmerist

Tidebinder Mage

When Tidebinder Mage enters, 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.

Loxodon Warhammer

Equipped creature gets +3/+0 and has trample and lifelink.
Equip {3}

Sun Titan

Vigilance
Whenever Sun Titan enters or attacks, you may return target permanent card with mana value 3 or less from your graveyard to the battlefield.

Clever Impersonator

You may have Clever Impersonator enter as a copy of any nonland permanent on the battlefield.

Counterspell

Tideshaper Mystic

{T}: Target land becomes the basic land type of your choice until end of turn. Activate only during your turn.

Blackblade Reforged

Equipped creature gets +1/+1 for each land you control.
Equip legendary creature {3}
Equip {7}

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.)

Diviner's Wand

Equipped creature has "Whenever you draw a card, this creature gets +1/+1 and gains flying until end of turn" and "{4}: Draw a card."
Whenever a Wizard creature enters, you may attach Diviner's Wand to it.
Equip {3}

Dawn

Aftermath (Cast this spell only from your graveyard. Then exile it.)
Return all creature cards with power 2 or less from your graveyard to your hand.

Dusk

Destroy all creatures with power 3 or greater.

Dusk // Dawn

Silvergill Douser

{T}: Target creature gets -X/-0 until end of turn, where X is the number of Merfolk and/or Faeries you control.

Aquitect's Will

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.

Streambed Aquitects

{T}: Target Merfolk creature gets +1/+1 and gains islandwalk until end of turn. (It can't be blocked as long as defending player controls an Island.)
{T}: Target land becomes an Island until end of turn.

Fireshrieker

Equipped creature has double strike. (It deals both first-strike and regular combat damage.)
Equip {2} ({2}: Attach to target creature you control. Equip only as a sorcery.)

Drowner of Secrets

Tap an untapped Merfolk you control: Target player mills a card.

Sygg, River Guide

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.

Throne of the God-Pharaoh

At the beginning of your end step, each opponent loses life equal to the number of tapped creatures you control.

Sage's Dousing

Counter target spell unless its controller pays {3}. If you control a Wizard, draw a card.

Judge of Currents

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

Godsend

Equipped creature gets +3/+3.
Whenever equipped creature blocks or becomes blocked by one or more creatures, you may exile one of those creatures.
Your opponents can't cast spells with the same name as a card exiled with Godsend.
Equip {3}

Wanderwine Hub

As Wanderwine Hub enters, you may reveal a Merfolk card from your hand. If you don't, Wanderwine Hub enters tapped.
{T}: Add {W} or {U}.

Merrow Commerce

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

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.

Sage of Fables

Each other Wizard creature you control enters with an additional +1/+1 counter on it.
{2}, Remove a +1/+1 counter from a creature you control: Draw a card.

Mirror Entity

Changeling (This card is every creature type.)
{X}: Until end of turn, creatures you control have base power and toughness X/X and gain all creature types.

Harpoon Sniper

{W}, {T}: Harpoon Sniper deals X damage to target attacking or blocking creature, where X is the number of Merfolk you control.

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.

Merfolk Looter

Lord of Atlantis

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