Ghostly Flicker

Exile two target artifacts, creatures, and/or lands you control, then return those cards to the battlefield under your control.

Triton Shorestalker

Nyxborn Triton

Bestow {4}{U} (If you cast this card for its bestow cost, it's an Aura spell with enchant creature. It becomes a creature again if it's not attached to a creature.)
Enchanted creature gets +2/+3.

Negate

Staff of the Mind Magus

Whenever you cast a blue spell or an Island enters the battlefield under your control, you gain 1 life.

Deny Existence

Counter target creature spell. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Essence Scatter

Merfolk Spy

Islandwalk (This creature can't be blocked as long as defending player controls an Island.)
Whenever Merfolk Spy deals combat damage to a player, that player reveals a card at random from their hand.

Cancel

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.

Towering-Wave Mystic

Whenever Towering-Wave Mystic deals damage, target player mills that many cards.

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}.)

Island

Merfolk Skyscout

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

Turtleshell Changeling

Changeling (This card is every creature type.)
{1}{U}: Switch Turtleshell Changeling's power and toughness until end of turn.

Unsummon

Return target creature to its owner's hand.

Switcheroo

Exchange control of two target creatures.

Nullify

Vodalian Arcanist

{T}: Add {C}. Spend this mana only to cast an instant or sorcery spell.

Sworn Guardian

Maritime Guard

Desert of the Mindful

Desert of the Mindful enters the battlefield tapped.
{T}: Add {U}.
Cycling {1}{U} ({1}{U}, Discard this card: Draw a card.)

Verity Circle

Whenever a creature an opponent controls becomes tapped, if it isn't being declared as an attacker, you may draw a card.
{4}{U}: Tap target creature without flying.

Deepwater Hypnotist

Inspired — Whenever Deepwater Hypnotist becomes untapped, target creature an opponent controls gets -3/-0 until end of turn.

Spell Pierce

Counter target noncreature spell unless its controller pays {2}.

Littjara Kinseekers

Changeling (This card is every creature type.)
When Littjara Kinseekers enters the battlefield, if you control three or more creatures that share a creature type, put a +1/+1 counter on Littjara Kinseekers, then scry 1.

Mizzium Skin

Target creature you control gets +0/+1 and gains hexproof until end of turn.
Overload {1}{U} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")

Moonlit Scavengers

When Moonlit Scavengers enters the battlefield, if you control an artifact or enchantment, return target creature an opponent controls to its owner's hand.

Raving Visionary

{U}, {T}: Draw a card, then discard a card.
Delirium — {2}{U}, {T}: Draw a card. Activate only if there are four or more card types among cards in your graveyard.

Wind Strider

Annul

Counter target artifact or enchantment spell.

Shaper Apprentice

Shaper Apprentice has flying as long as you control another Merfolk.

Merfolk Looter

Venture Deeper

Target player mills four cards. (Then exile this card. You may cast the creature later from exile.)

Merfolk Secretkeeper

Merfolk Secretkeeper // Venture Deeper

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.)