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

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

Whenever Ragavan, Nimble Pilferer deals combat damage to a player, create a Treasure token and exile the top card of that player's library. Until end of turn, you may cast that card. Dash {1}{R} (You may cast this spell for its dash cost. If you do, it gains haste, and it's returned from the battlefield to its owner's hand at the beginning of the next end step.)

When Shoreline Scout enters the battlefield, you may exile a Merfolk card or a land card from your hand. If you do, conjure a card named Tropical Island into your hand. As long as another Merfolk or an Island entered the battlefield under your control this turn, Shoreline Scout gets +1/+0.

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}.)

Nonbasic lands are Mountains.

Draw three cards, then put two cards from your hand on top of your library in any order.

Counter target spell.

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}, Pay 1 life: Add {U} or {R}. {1}, {T}, Sacrifice Fiery Islet: Draw a card.

{T}, Pay 1 life, Sacrifice Flooded Strand: Search your library for a Plains or Island card, put it onto the battlefield, then shuffle.

({T}: Add {U}.)

{T}: Add {C}. {1}: Mutavault becomes a 2/2 creature with all creature types until end of turn. It's still a land.

{T}, Pay 1 life, Sacrifice Polluted Delta: Search your library for an Island or Swamp card, put it onto the battlefield, then shuffle.

Choose target spell or permanent that's red or green. Its owner puts it on the top or bottom of their library.

Nonbasic lands are Mountains.

Choose one — • Brotherhood's End deals 3 damage to each creature and each planeswalker. • Destroy all artifacts with mana value 3 or less.

As Pithing Needle enters the battlefield, choose a card name. Activated abilities of sources with the chosen name can't be activated unless they're mana abilities.

When Soul-Guide Lantern enters the battlefield, exile target card from a graveyard. {T}, Sacrifice Soul-Guide Lantern: Exile each opponent's graveyard. {1}, {T}, Sacrifice Soul-Guide Lantern: Draw a card.