Other Merfolk get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Other Merfolk creatures you control get +1/+1 and have islandwalk. (They can't be blocked as long as defending player controls an Island.)

Protection from red Elemental creatures you control get +1/+1. When Master of Waves enters the battlefield, create a number of 1/0 blue Elemental creature tokens equal to your devotion to blue. (Each {U} in the mana costs of permanents you control counts toward your devotion to blue.)

Flash When Merfolk Trickster enters the battlefield, tap target creature an opponent controls. It loses all abilities until end of turn.

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

Flash Flying When Subtlety enters the battlefield, choose up to one target creature spell or planeswalker spell. Its owner puts it on the top or bottom of their library. Evoke—Exile a blue card from your hand.

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}.)

As Cavern of Souls enters the battlefield, choose a creature type. {T}: Add {C}. {T}: Add one mana of any color. Spend this mana only to cast a creature spell of the chosen type, and that spell can't be countered.

({T}: Add {U}.)

{T}: Add {U}. {U}, {T}: Untap target legendary permanent.

{T}: Add {U}. {1}: Return Oboro, Palace in the Clouds to its owner's hand.

{T}: Add {U}. Channel — {3}{U}, Discard Otawara, Soaring City: Return target artifact, creature, enchantment, or planeswalker to its owner's hand. This ability costs {1} less to activate for each legendary creature you control.