Have

Elrond, Lord of Rivendell

Whenever Elrond or another creature you control enters, scry 1. If this is the second time this ability has resolved this turn, the Ring tempts you.

Merfolk Observer

When this creature enters, look at the top card of target player's library.

Shiva, Warden of Ice

(As this Saga enters and after your draw step, add a lore counter.)
I, II — Mesmerize — Target creature can't be blocked this turn.
III — Cold Snap — Tap all lands your opponents control. Exile Shiva, then return it to the battlefield (front face up).

Jill, Shiva's Dominant

When Jill enters, return up to one other target nonland permanent to its owner's hand.
{3}{U}{U}, {T}: Exile Jill, then return it to the battlefield transformed under its owner's control. Activate only as a sorcery.

Jill, Shiva's Dominant // Shiva, Warden of Ice

Y'shtola Rhul

At the beginning of your end step, exile target creature you control, then return it to the battlefield under its owner's control. Then if it's the first end step of the turn, there is an additional end step after this step.

Hydro-Man, Fluid Felon

Whenever you cast a blue spell, if Hydro-Man is a creature, he gets +1/+1 until end of turn.
At the beginning of your end step, untap Hydro-Man. Until your next turn, he becomes a land and gains "{T}: Add {U}." (He's not a creature during that time.)

Skywarp Skaab

Flying
When this creature enters, you may exile two creature cards from your graveyard. If you do, draw a card.

Hide on the Ceiling

Exile X target artifacts and/or creatures. Return the exiled cards to the battlefield under their owners' control at the beginning of the next end step.

Ultimecia, Temporal Threat

When Ultimecia enters, tap all creatures your opponents control.
Whenever a creature you control deals combat damage to a player, draw a card.

Spider-Byte, Web Warden

When Spider-Byte enters, return up to one target nonland permanent to its owner's hand.

Blur

Exile target creature you control, then return that card to the battlefield under its owner's control.
Draw a card.

Humbling Elder

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

Micromancer

When this creature enters, you may search your library for an instant or sorcery card with mana value 1, reveal it, put it into your hand, then shuffle.