Cavern of Souls

As Cavern of Souls enters the battlefield, choose a creature type. {T}: Add {1} to your mana pool. {T}: Add one mana of any color to your mana pool. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

Phantasmal Image

You may have Phantasmal Image enter the battlefield as a copy of any creature on the battlefield, except it's an Illusion in addition to its other types and it gains "When this creature becomes the target of a spell or ability, sacrifice it."

Master of the Pearl Trident

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They are unblockable as long as defending player controls an Island.)

Harbinger of the Tides

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.

Kira, Great Glass-Spinner

Flying Creatures you control have "Whenever this creature becomes the target of a spell or ability for the first time in a turn, counter that spell or ability."

Aether Vial

At the beginning of your upkeep, you may put a charge counter on ?ther Vial. {T}: You may put a creature card with converted mana cost equal to the number of charge counters on ?ther Vial from your hand onto the battlefield.

Spellskite

{U/P}: Change a target of target spell or ability to Spellskite. ({U/P} can be paid with either {U} or 2 life.)

Mutavault

{T}: Add {1} to your mana pool. {1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.

Merrow Reejerey

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

Cryptic Command

Oboro, Palace in the Clouds

{T}: Add {U} to your mana pool. {1}: Return Oboro, Palace in the Clouds to its owner's hand.

Cursecatcher

Sacrifice Cursecatcher: Counter target instant or sorcery spell unless its controller pays {1}.

Sygg, River Cutthroat

At end of turn, if an opponent lost 3 or more life this turn, you may draw a card. (Damage causes loss of life.)

Master of Waves

Protection from red Elemental creatures you control get +1/+1. When Master of Waves enters the battlefield, put a number of 1/0 blue Elemental creature tokens onto the battlefield equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Lord of Atlantis

All Merfolk in play gain islandwalk and +1/+1 while this card is in play.

Spreading Seas

Enchant land When Spreading Seas enters the battlefield, draw a card. Enchanted land is an Island.

Kopala, Warden of Waves

1UU

Silvergill Adept

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.