msem-instigator

Cast this spell only if you have 10 or less life.
Flying, trample

At the beginning of your upkeep, exile the top card of your library. You may play it this turn. Whenever you cast an instant or sorcery spell exiled this way, you may copy it and you may choose new targets for the copy.

This spell costs {1} less to cast for each foretold card you own in exile, each permanent you control with foretell, and each card with foretell in your graveyard.
Flying
When Tyrant of Prophecies enters, it deals 3 damage to any target.

Tyrant of the Depths has hexproof as long as it's untapped.
Whenever you cycle Tyrant of the Depths or it attacks, tap target nonland permanent you don't control and put a stun counter on it.
Basic landcycling {1}{U} ({1}{U}, Discard this card: Search your library for a basic land card, reveal it, and put it into your hand, then shuffle.)

Flying, vigilance
When Tyriel, Angelic Artisan enters, you may search your library for an Equipment card, reveal it, put it into your hand, then shuffle.
Whenever Tyriel attacks, you may put an Equipment card from your hand onto the battlefield, then attach it to Tyriel.

Whenever Ujuraq, Elder Watcher enters or attacks, surveil 1. (Look at the top card of your library. You may put that card into your graveyard.)
Ujuraq gets +3/+3 and has trample as long as there are eight or more cards in your graveyard.

Vision (Whenever this or another creature you control enters, reveal the top card of your library. Then you may put that card into your graveyard.)
Whenever Ulek Crusher envisions a creature card, it gets +2/+2 until end of turn.

Cast this spell only if you control a creature.
Exile all creatures.

Flying
{1}{R}{R}{R}: Return Umbral Hunter from your graveyard to your hand. If it's the declare attackers step, up to one target attacking creature gets +3/+1 until end of turn.

{2/B}: Umbral Stalker gains indestructible until end of turn. Tap it.

Resolute — Umbral Wolf can't be blocked by more than one creature as long as Umbral Wolf's power is greater than its base power.

Assault {X}{R}{R} (Whenever this attacks, if it has no +1/+1 counters on it, you may pay its assault cost to put a +1/+1 counter on it.)
Other Elementals you control get +1/+0.
When Unbound Annihilus assaults, create X 1/1 red Elemental creature tokens that are tapped and attacking.

Whenever you cast another creature spell, create a 1/1 green and white Elk Spirit creature token.
As long as you control five or more Elk, creatures you control get +3/+3 and have trample.

Target opponent sacrifices half the permanents they control, rounded down.

Search your library for a permanent card with mana value X or less, reveal it, put it into your hand, then shuffle.

Hexproof
When Uncharted Leviathan enters, target creature can't be blocked this turn.

When Uncovered Secrets enters, search your library for an artifact card, reveal that card, then shuffle and put that card on top.
Concord — Creatures you control get +1/+1 as long as there are four or more card types among permanents you control.

When Undead Archivist enters, return target instant or sorcery card from your graveyard to your hand.
Revive {5}{U} ({5}{U}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

Creature cards in your graveyard have revive {2}{B}{B}. ({2}{B}{B}, Exile this card from your graveyard: Create a token that's a copy of it except it has no mana cost. Revive only as a sorcery.)

You may have Undecided Mimic enter or phase in as a copy of any creature on the battlefield, except it has this ability and “{2}{U}{U}: Undecided Mimic phases out.” (Treat it and anything attached to it as though they don't exist until its controller's next turn.)

As an additional cost to cast this spell, exile a creature card from your graveyard.
You draw three cards and you lose 3 life.

When Undying Lich dies, create a 6/3 black Zombie Horror creature token with menace and “When this creature dies, create an 8/5 black Elder Demon Horror creature token with flying, deathtouch, and menace.”

When Undying Watcher enters, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)
Sacrifice another creature: Undying Watcher gains indestructible until end of turn.

Create three 1/1 colorless Servo artifact creature tokens.
Exile Unending Armies. At the beginning of your upkeep, if you have more life than each opponent, you may cast it without paying its mana cost. Otherwise, put it into your graveyard.

Whenever one or more creatures you control deal combat damage to a player, that player discards a card. Then that player may discard another card. If they don't, create two 2/1 black Nightmare creature tokens with “This creature can't block.”
Fleeting {1}{B}{B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)