Concentrate

Threads of Disloyalty

Enchant creature with mana value 2 or less
You control enchanted creature.

River Sneak

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

Forest

Vow of Flight

Enchant creature
Enchanted creature gets +2/+2, has flying, and can't attack you or planeswalkers you control.

Tempest Caller

When Tempest Caller enters, tap all creatures target opponent controls.

Prey Upon

Target creature you control fights target creature you don't control. (Each deals damage equal to its power to the other.)

Waker of the Wilds

{X}{G}{G}: Put X +1/+1 counters on target land you control. That land becomes a 0/0 Elemental creature with haste. It's still a land.

Time Warp

Target player takes an extra turn after this one.

Island

Kumena's Speaker

Kumena's Speaker gets +1/+1 as long as you control another Merfolk or an Island.

Soul of the Harvest

Trample
Whenever another nontoken creature you control enters, you may draw a card.

Deeproot Warrior

Whenever Deeproot Warrior becomes blocked, it gets +1/+1 until end of turn.

Merfolk Sovereign

Other Merfolk creatures you control get +1/+1.
{T}: Target Merfolk creature can't be blocked this turn.

Jungle Barrier

Defender (This creature can't attack.)
When Jungle Barrier enters, draw a card.

Watertrap Weaver

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

Shaper Apprentice

Shaper Apprentice has flying as long as you control another Merfolk.

Aether Gale

Return six target nonland permanents to their owners' hands.

Giant Growth

Target creature gets +3/+3 until end of turn.

Unsummon

Return target creature to its owner's hand.

Vineshaper Mystic

When Vineshaper Mystic enters, put a +1/+1 counter on each of up to two target Merfolk you control.

Rancor

Enchant creature
Enchanted creature gets +2/+0 and has trample.
When Rancor is put into a graveyard from the battlefield, return Rancor to its owner's hand.

Unknown Shores

{T}: Add {C}.
{1}, {T}: Add one mana of any color.

Prismatic Lens

Air Elemental

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.