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.

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.

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.

Vapor Snag

Return target creature to its owner's hand. Its controller loses 1 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.

Unified Will

Counter target spell if you control more creatures than that spell's controller.

Relic of Progenitus

{T}: Target player removes a card in his or her graveyard from the game. {1}, Remove Relic of Progenitus from the game: Remove all graveyards from the game. Draw a card.

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.

Misty Rainforest

Natural State

Botanical Sanctum

Spell Pierce

U

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.

Merfolk Mistbinder

Other Merfolk you control get +1/+1.

Benthic Biomancer

{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.

Breeding Pool

({T}: Add {G} or {U}.) As Breeding Pool enters the battlefield, you may pay 2 life. If you don't, it enters the battlefield tapped.

Island

({T}: Add {U}.)