Counterspell

Distant Melody

Choose a creature type. Draw a card for each permanent you control of that type.

Vodalian Hexcatcher

Flash
Other Merfolk you control get +1/+1.
Sacrifice a Merfolk: Counter target noncreature spell unless its controller pays {1}.

River's Rebuke

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

Rapid Hybridization

Destroy target creature. It can't be regenerated. That creature's controller creates a 3/3 green Frog Lizard creature token.

Hindering Light

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

Sky Diamond

Riptide Laboratory

{T}: Add {C}.
{1}{U}, {T}: Return target Wizard you control to its owner's hand.

Time Wipe

Return a creature you control to its owner's hand, then destroy all creatures.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

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}.)

Folk Hero

Commander creatures you own have "Whenever you cast a spell that shares a creature type with this creature, draw a card. This ability triggers only once each turn."

Marble Diamond

Marble Diamond enters tapped.
{T}: Add {W}.

Talisman of Progress

{T}: Add {C}.
{T}: Add {W} or {U}. Talisman of Progress deals 1 damage to you.

Generous Gift

Destroy target permanent. Its controller creates a 3/3 green Elephant creature token.

Path to Exile

Exile target creature. Its controller may search their library for a basic land card, put that card onto the battlefield tapped, then shuffle.