Have

Dirge Bat

Mutate {4}{B}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Flying
Whenever this creature mutates, destroy target creature or planeswalker an opponent controls.

Island

Brineborn Cutthroat

Flash (You may cast this spell any time you could cast an instant.)
Whenever you cast a spell during an opponent's turn, put a +1/+1 counter on this creature.

Shipwreck Marsh

Shipwreck Marsh enters tapped unless you control two or more other lands.
{T}: Add {U} or {B}.

Sea-Dasher Octopus

Mutate {1}{U} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Whenever this creature deals combat damage to a player, draw a card.

Slitherwisp

Flash
Whenever you cast another spell that has flash, you draw a card and each opponent loses 1 life.

Watery Grave

({T}: Add {U} or {B}.)
As Watery Grave enters, you may pay 2 life. If you don't, it enters tapped.

Sunken Hollow

({T}: Add {U} or {B}.)
Sunken Hollow enters tapped unless you control two or more basic lands.

Disdainful Stroke

Counter target spell with mana value 4 or greater.

Negate

Cunning Nightbonder

Flash
Spells with flash you cast cost {1} less to cast and can't be countered.

Heartless Act

Choose one —
• Destroy target creature with no counters on it.
• Remove up to three counters from target creature.

Fatal Push

Destroy target creature if it has mana value 2 or less.
Revolt — Destroy that creature if it has mana value 4 or less instead if a permanent you controlled left the battlefield this turn.

Rewind

Counter target spell. Untap up to four lands.

Sheoldred, the Apocalypse

Deathtouch
Whenever you draw a card, you gain 2 life.
Whenever an opponent draws a card, they lose 2 life.

Spectral Sailor

Flash (You may cast this spell any time you could cast an instant.)
Flying
{3}{U}: Draw a card.