Watcher for Tomorrow

Hideaway 4 (When this creature enters the battlefield, look at the top four cards of your library, exile one face down, then put the rest on the bottom in a random order.)
Watcher for Tomorrow enters the battlefield tapped.
When Watcher for Tomorrow leaves the battlefield, put the exiled card into its owner's hand.

Day of the Dragons

When Day of the Dragons enters the battlefield, exile all creatures you control. Then create that many 5/5 red Dragon creature tokens with flying.
When Day of the Dragons leaves the battlefield, sacrifice all Dragons you control. Then return the exiled cards to the battlefield under your control.

Curator's Ward

Enchant permanent
Enchanted permanent has hexproof.
When enchanted permanent leaves the battlefield, if it was historic, draw two cards. (Artifacts, legendaries, and Sagas are historic.)

Toothy, Imaginary Friend

Partner with Pir, Imaginative Rascal (When this creature enters the battlefield, target player may put Pir into their hand from their library, then shuffle.)
Whenever you draw a card, put a +1/+1 counter on Toothy, Imaginary Friend.
When Toothy leaves the battlefield, draw a card for each +1/+1 counter on it.

Floodgate

Defender (This creature can't attack.)
When Floodgate has flying, sacrifice it.
When Floodgate leaves the battlefield, it deals damage equal to half the number of Islands you control, rounded down, to each nonblue creature without flying.

Wormfang Manta

Flying
When Wormfang Manta enters the battlefield, you skip your next turn.
When Wormfang Manta leaves the battlefield, you take an extra turn after this one.

Wormfang Drake

Flying
When Wormfang Drake enters the battlefield, sacrifice it unless you exile a creature you control other than Wormfang Drake.
When Wormfang Drake leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Giant Oyster

You may choose not to untap Giant Oyster during your untap step.
{T}: For as long as Giant Oyster remains tapped, target tapped creature doesn't untap during its controller's untap step, and at the beginning of each of your draw steps, put a -1/-1 counter on that creature. When Giant Oyster leaves the battlefield or becomes untapped, remove all -1/-1 counters from the creature.

Slithermuse

When Slithermuse leaves the battlefield, choose an opponent. If that player has more cards in hand than you, draw cards equal to the difference.
Evoke {3}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters the battlefield.)

Aligned Hedron Network

When Aligned Hedron Network enters the battlefield, exile all creatures with power 5 or greater until Aligned Hedron Network leaves the battlefield. (Those creatures return under their owners' control.)

Wormfang Newt

When Wormfang Newt enters the battlefield, exile a land you control.
When Wormfang Newt leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Nevermaker

Flying
When Nevermaker leaves the battlefield, put target nonland permanent on top of its owner's library.
Evoke {3}{U} (You may cast this spell for its evoke cost. If you do, it's sacrificed when it enters the battlefield.)

Icy Prison

When Icy Prison enters the battlefield, exile target creature.
At the beginning of your upkeep, sacrifice Icy Prison unless any player pays {3}.
When Icy Prison leaves the battlefield, return the exiled card to the battlefield under its owner's control.

The Ozolith

Whenever a creature you control leaves the battlefield, if it had counters on it, put those counters on The Ozolith.
At the beginning of combat on your turn, if The Ozolith has counters on it, you may move all counters from The Ozolith onto target creature.

Parallax Tide

Fading 5 (This enchantment enters the battlefield with five fade counters on it. At the beginning of your upkeep, remove a fade counter from it. If you can't, sacrifice it.)
Remove a fade counter from Parallax Tide: Exile target land.
When Parallax Tide leaves the battlefield, each player returns to the battlefield all cards they own exiled with Parallax Tide.

Wormfang Turtle

When Wormfang Turtle enters the battlefield, exile a land you control.
When Wormfang Turtle leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Thassa, Deep-Dwelling

Indestructible
As long as your devotion to blue is less than five, Thassa isn't a creature.
At the beginning of your end step, exile up to one other target creature you control, then return that card to the battlefield under your control.
{3}{U}: Tap another target creature.

Thalakos Seer

Shadow (This creature can block or be blocked by only creatures with shadow.)
When Thalakos Seer leaves the battlefield, draw a card.