Mutavault

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

Relic of Progenitus

{T}: Target player exiles a card from their graveyard.
{1}, Exile Relic of Progenitus: Exile all graveyards. Draw a card.

Darkslick Shores

Darkslick Shores enters tapped unless you control two or fewer other lands.
{T}: Add {U} or {B}.

Spellskite

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

Harbinger of the Tides

You may cast this card 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 this creature enters, you may return target tapped creature an opponent controls to its owner's hand.

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.)

Kira, Great Glass-Spinner

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

Vapor Snag

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

Phantasmal Image

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

Spell Pierce

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

Dismember

({B/P} can be paid with either {B} or 2 life.)
Target creature gets -5/-5 until end of turn.

Cursecatcher

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

Merrow Reejerey

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

Island

Hibernation

Return all green permanents to their owners' hands.

Spreading Seas

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

Silvergill Adept

As an additional cost to cast this spell, reveal a Merfolk card from your hand or pay {3}.
When Silvergill Adept enters, draw a card.

Lord of Atlantis

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

Master of Waves

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

Hurkyl's Recall

Return all artifacts target player owns to their hand.

Vedalken Shackles

You may choose not to untap Vedalken Shackles during your untap step.
{2}, {T}: Gain control of target creature with power less than or equal to the number of Islands you control for as long as Vedalken Shackles remains tapped.

Cavern of Souls

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

Aether Vial

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