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 Brineborn Cutthroat.

Swamp

Blacklance Paragon

Flash
When Blacklance Paragon enters the battlefield, target Knight gains deathtouch and lifelink until end of turn.

Ashiok's Erasure

Flash
When Ashiok's Erasure enters the battlefield, exile target spell.
Your opponents can't cast spells with the same name as the exiled card.
When Ashiok's Erasure leaves the battlefield, return the exiled card to its owner's hand.

Heartless Act

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

Boomerang

Return target permanent to its owner's hand.

Cunning Nightbonder

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

Island

Mana Leak

Counter target spell unless its controller pays {3}.

Temple of Deceit

Temple of Deceit enters the battlefield tapped.
When Temple of Deceit enters the battlefield, scry 1.
{T}: Add {U} or {B}.

Castle Vantress

Castle Vantress enters the battlefield tapped unless you control an Island.
{T}: Add {U}.
{2}{U}{U}, {T}: Scry 2.

Slitherwisp

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