Cards/Merfolk

Whenever one or more +1/+1 counters are put on Benthic Biomancer, draw a card, then discard a card.

Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on Brineborn Cutthroat.

, : Move a +1/+1 counter from target creature you control onto another target creature you control.

Equipped creatures you control have flying and haste.

When Emry, Lurker of the Loch enters the battlefield, mill four cards.
: Choose target artifact card in your graveyard. You may cast that card this turn. (You still pay its costs. Timing rules still apply.)

Whenever another Merfolk enters the battlefield under your control, put a +1/+1 counter on Forerunner of the Heralds.

When Jade Guardian enters the battlefield, put a +1/+1 counter on target Merfolk you control.

Abilities your opponents activate that target a Merfolk you control cost more to activate.

Tap three untapped Merfolk you control: Draw a card.
Tap five untapped Merfolk you control: Put a +1/+1 counter on each Merfolk you control.

When Merfolk Skydiver enters the battlefield, put a +1/+1 counter on target creature you control.
: Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

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

Whenever you cast a Merfolk spell, you may tap or untap target permanent.

When Mire Triton enters the battlefield, mill two cards and you gain 2 life.

Whenever another Merfolk enters the battlefield under your control, River Sneak gets +1/+1 until end of turn.

When Sentinel of the Pearl Trident enters the battlefield, you may exile target historic permanent you control. If you do, return that card to the battlefield under its owner's control at the beginning of the next end step. (Artifacts, legendaries, and Sagas are historic.)

, Remove a +1/+1 counter from a creature you control: Draw a card.

When Shipwreck Dowser enters the battlefield, return target instant or sorcery card from your graveyard to your hand.

When Silvergill Adept enters the battlefield, draw a card.

When Storm Sculptor enters the battlefield, return a creature you control to its owner's hand.

When Swift Warden enters the battlefield, target Merfolk you control gains hexproof until end of turn. (It can't be the target of spells or abilities your opponents control.)

Swimmer in Nightmares can't be blocked as long as you control an Ashiok planeswalker.

When Teferi's Wavecaster enters the battlefield, you may search your library and/or graveyard for a card named Teferi, Timeless Voyager, reveal it, and put it into your hand. If you search your library this way, shuffle it.

You have no maximum hand size.
When Tishana enters the battlefield, draw a card for each creature you control.

Flying

: Wishful Merfolk loses defender and becomes a Human until end of turn.

When World Shaper dies, return all land cards from your graveyard to the battlefield tapped.

: 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.