Brave the Sands

Creatures you control have vigilance.
Each creature you control can block an additional creature each combat.

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.

Herald of Secret Streams

Creatures you control with +1/+1 counters on them can't be blocked.

Detention Sphere

When Detention Sphere enters, you may exile target nonland permanent not named Detention Sphere and all other permanents with the same name as that permanent.
When Detention Sphere leaves the battlefield, return the exiled cards to the battlefield under their owner's control.

Dovin Baan

+1: Until your next turn, up to one target creature gets -3/-0 and its activated abilities can't be activated.
−1: You gain 2 life and draw a card.
−7: You get an emblem with "Your opponents can't untap more than two permanents during their untap steps."

Harpoon Sniper

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

Wall of Denial

Defender, flying
Shroud (This creature can't be the target of spells or abilities.)

Vodalian Wave-Knight

Whenever you draw a card, put a +1/+1 counter on each other Merfolk and/or Knight you control.

Island

Vengeful Archon

Flying
{X}: Prevent the next X damage that would be dealt to you this turn. If damage is prevented this way, Vengeful Archon deals that much damage to target player or planeswalker.

Merrow Reejerey

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

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}.

Plains

Adarkar Valkyrie

Flying, vigilance
{T}: When target creature other than Adarkar Valkyrie dies this turn, return that card to the battlefield under your control.

Wave Goodbye

Return each creature without a +1/+1 counter on it to its owner's hand.

Sejiri Refuge

Sejiri Refuge enters tapped.
When Sejiri Refuge enters, you gain 1 life.
{T}: Add {W} or {U}.

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}.)

Azorius Chancery

Azorius Chancery enters tapped.
When Azorius Chancery enters, return a land you control to its owner's hand.
{T}: Add {W}{U}.

Silvergill Douser

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