River Heralds' Boon

Put a +1/+1 counter on target creature and a +1/+1 counter on up to one target Merfolk.

Merfolk Mistbinder

Tempest Caller

When this creature enters, tap all creatures target opponent controls.

Herald of Secret Streams

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

Kopala, Warden of Waves

Spells your opponents cast that target a Merfolk you control cost {2} more to cast.
Abilities your opponents activate that target a Merfolk you control cost {2} more to activate.

Merfolk Trickster

Flash
When this creature enters, tap target creature an opponent controls. It loses all abilities until end of turn.

Dive Down

Target creature you control gets +0/+3 and gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Riverwise Augur

When this creature enters, draw three cards, then put two cards from your hand on top of your library in any order.

Deeproot Waters

Whenever you cast a Merfolk spell, create a 1/1 blue Merfolk creature token with hexproof. (A creature with hexproof can't be the target of spells or abilities your opponents control.)

Forest

Tatyova, Benthic Druid

Landfall — Whenever a land you control enters, you gain 1 life and draw a card.

Scry 1. (Look at the top card of your library. You may put that card on the bottom.)
Draw a card.

Silvergill Adept

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

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 this creature, draw a card, then discard a card.

Wizard's Retort

This spell costs {1} less to cast if you control a Wizard.
Counter target spell.

Seafloor Oracle

Whenever a Merfolk you control deals combat damage to a player, draw a card.

Merfolk Branchwalker

When this creature enters, it explores. (Reveal the top card of your library. Put that card into your hand if it's a land. Otherwise, put a +1/+1 counter on this creature, then put the card back or put it into your graveyard.)

Spell Pierce

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

Chemister's Insight

Draw two cards.
Jump-start (You may cast this card from your graveyard by discarding a card in addition to paying its other costs. Then exile this card.)

Shapers of Nature

{3}{G}: Put a +1/+1 counter on target creature.
{2}{U}, Remove a +1/+1 counter from a creature you control: Draw a card.

Watertrap Weaver

When this creature enters, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Deeproot Champion

Whenever you cast a noncreature spell, put a +1/+1 counter on this creature.

Island

Mistcaller

Sacrifice this creature: Until end of turn, if a nontoken creature would enter and it wasn't cast, exile it instead.

Forerunner of the Heralds

When this creature enters, you may search your library for a Merfolk card, reveal it, then shuffle and put that card on top.
Whenever another Merfolk you control enters, put a +1/+1 counter on this creature.

Zegana, Utopian Speaker

When Zegana enters, if you control another creature with a +1/+1 counter on it, draw a card.
{4}{G}{U}: Adapt 4. (If this creature has no +1/+1 counters on it, put four +1/+1 counters on it.)
Each creature you control with a +1/+1 counter on it has trample.