Unblinking Observer

{T}: Add {U}. Spend this mana only to pay a disturb cost or cast an instant or sorcery spell.

Brinebarrow Intruder

Flash
When Brinebarrow Intruder enters, target creature an opponent controls gets -2/-0 until end of turn.

Bury in Books

This spell costs {2} less to cast if it targets an attacking creature.
Put target creature into its owner's library second from the top.

Curate

Surveil 2. (Look at the top two cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)
Draw a card.

Undersea Invader

Flash (You may cast this spell any time you could cast an instant.)
Undersea Invader enters tapped.

Island

Frost Trickster

Flying
When Frost Trickster enters, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.

Syncopate

Counter target spell unless its controller pays {X}. If that spell is countered this way, exile it instead of putting it into its owner's graveyard.

Wanderlight Spirit

Flying
Wanderlight Spirit can block only creatures with flying.

Spell Pierce

Counter target noncreature spell unless its controller pays {2}.

Network Disruptor

Flying
When Network Disruptor enters, tap target permanent.