Planar Incision

Exile target artifact or creature, then return it to the battlefield under its owner's control with a +1/+1 counter on it.

River Sneak

This creature can't be blocked.
Whenever another Merfolk you control enters, this creature gets +1/+1 until end of turn.

Stasis Field

Enchant creature
Enchanted creature has base power and toughness 0/2, has defender, and loses all other abilities.

River's Rebuke

Return all nonland permanents target player controls to their owner's hand.

Island

Consider

Surveil 1. (Look at the top card of your library. You may put it into your graveyard.)
Draw a card.

Herald of Secret Streams

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

Jungle Delver

{3}{G}: Put a +1/+1 counter on this creature.

Mist-Cloaked Herald

Crashing Tide

This spell has flash as long as you control a Merfolk.
Return target creature to its owner's hand.
Draw a card.

Fading Hope

Return target creature to its owner's hand. If its mana value was 3 or less, scry 1. (Look at the top card of your library. You may put that card on the bottom.)

Keep Safe

Counter target spell that targets a permanent you control.
Draw a card.

Thornwood Falls

This land enters tapped.
When this land enters, you gain 1 life.
{T}: Add {G} or {U}.

Stony Strength

Put a +1/+1 counter on target creature you control. Untap that creature.

Moment of Truth

Look at the top three cards of your library. Put one of those cards into your hand, one into your graveyard, and one on the bottom of your library.

Reconnaissance Mission

Whenever a creature you control deals combat damage to a player, you may draw a card.
Cycling {2} ({2}, Discard this card: Draw a card.)

Dreamroot Cascade

This land enters tapped unless you control two or more other lands.
{T}: Add {G} or {U}.

Jade Bearer

When this creature enters, put a +1/+1 counter on another target Merfolk you control.

Atraxa's Fall

Destroy target artifact, battle, enchantment, or creature with flying.

Growth Spiral

Draw a card. You may put a land card from your hand onto the battlefield.

Jungleborn Pioneer

When this creature enters, create a 1/1 blue Merfolk creature token with hexproof. (It can't be the target of spells or abilities your opponents control.)

Nykthos, Shrine to Nyx

{T}: Add {C}.
{2}, {T}: Choose a color. Add an amount of mana of that color equal to your devotion to that color. (Your devotion to a color is the number of mana symbols of that color in the mana costs of permanents you control.)

Song of Freyalise

(As this Saga enters and after your draw step, add a lore counter. Sacrifice after III.)
I, II — Until your next turn, creatures you control gain "{T}: Add one mana of any color."
III — Put a +1/+1 counter on each creature you control. Those creatures gain vigilance, trample, and indestructible until end of turn.

The Hunter Maze

This land enters tapped.
{T}: Add {G}.
{1}{G}, {T}, Sacrifice this land: Draw a card.

Uncharted Haven

This land enters tapped. As it enters, choose a color.
{T}: Add one mana of the chosen color.

One With the Wind

Enchant creature
Enchanted creature gets +2/+2 and has flying.

Forest

Replicating Ring

{T}: Add one mana of any color.
At the beginning of your upkeep, put a night counter on this artifact. Then if it has eight or more night counters on it, remove all of them and create eight colorless snow artifact tokens named Replicated Ring with "{T}: Add one mana of any color."

Hunt the Weak

Put a +1/+1 counter on target creature you control. Then that creature fights target creature you don't control. (Each deals damage equal to its power to the other.)