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.

Unbound Spirit can't be blocked.
When you attain Unbound Spirit, tap target creature. That creature doesn't untap during its controller's next untap step.

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.

Whenever Unbudging Badger attacks, if its power is 4 or greater, it gains trample until end of turn.

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.

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.

Spend only mana produced by Treasures to cast spells.
{2}, Discard a card: Create two Treasure tokens.

Uncharted Haven enters tapped.
As Uncharted Haven enters, choose a color.
{T}: Add one mana of the chosen color.

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

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

When Uncovered Idol enters, scry 1.
{1}, Sacrifice Uncovered Idol: Put a +1/+1 counter on target creature. Activate only as a sorcery.

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.

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.)

Enchant creature
Enchanted creature gets +1/+0 and has menace. (That creature can't be blocked except by two or more creatures.)

Enchant land you control
Whenever enchanted land is tapped for mana, it produces an additional mana of that type.
{1}{G}, Sacrifice Under Untamed Skies: Destroy target creature with flying.

At the beginning of each opponent's upkeep, that player may discard their hand. If the player does, sacrifice Under the Gun. Otherwise, put a bullet counter on Under the Gun, then that player loses life equal to the number of bullet counters on it.

Concord — As long as there are four or more card types among permanents you control, you may cast the first spell you cast each turn without paying its mana cost.

Mark (Whenever this creature attacks, mark target creature until end of turn.)
Undercity Diplomat can't be blocked by marked creatures.

{T}, Sacrifice a permanent: Investigate. (Create a colorless Clue artifact token with “{2}, Sacrifice this artifact: Draw a card.”)