msem-instigator

Whenever you copy an instant or sorcery spell, you may pay {1}{U}. If you do, draw a card.
Whenever you cast an instant or sorcery spell, you may pay {1}{R}. If you do, Unbound Researchers deals 2 damage to target player or planeswalker.

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.

Up to two target creatures each get -3/-0 until end of turn. Tap those creatures. They don't untap during their controller's next untap step.

Destroy target artifact or enchantment. You gain life equal to its mana value.

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

When Unburdening Priest enters, you may exile any number of creatures you control not named Unburdening Priest, then return those cards to the battlefield under your control.

Target creature gets +1/+1 and gains double strike until end of turn. (That creature deals both first-strike and regular combat damage.)

Whenever Uncanny Allocator enters or attacks, give any number of permanents and/or players with counters on them another counter of each kind already there.

Put two -1/-1 counters on target creature.
Agonize (You may cast this card from your graveyard by putting a -1/-1 counter on a creature you control in addition to paying its other costs. Then exile this card.)

This spell costs {2} less to cast if you control a Scout.
Search your library for up to two basic land cards, reveal those cards, and put one onto the battlefield tapped and the other into your hand, then shuffle.

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

Creatures your opponents control get -2/-0 until end of turn.
Draw a card.

Whenever a creature you control is dealt damage, put a +1/+1 counter on that creature.

Enspirit (As you cast this spell, you may exile a nonenchantment creature you control until this leaves the battlefield or fails to resolve. Reduce this spell's cost by the exiled card's mana value.)
Deathtouch

Surveil 2, then draw three cards. (To 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.)

When Uncovered Mine enters and at the beginning of your upkeep, return a random card from your graveyard to your hand.

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.)

When Undead Banneret enters, if you control a legendary creature, each opponent loses 4 life.

This spell costs {1} less to cast for each Zombie creature card in your graveyard.
When Undead Harbinger enters, return target Zombie card from your graveyard to your hand.

When Undead Necromancer enters, search your library for a card, put that card into your graveyard, then shuffle.
As long as this card is in your graveyard, its name is Undead Servant.

When Undead Servant enters, create a 2/2 black Zombie creature token for each card named Undead Servant in your graveyard.

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.)

If you've scried this turn, this spell costs {2} less to cast and can't be countered.
Draw two cards.