msem-instigator

Harness (You may tap a creature you control as you cast this spell. If you do, reduce this spell's mana cost by that creature's power.)
Exile target nonland permanent an opponent controls. If this spell was harnessed by a token, you gain 5 life.

All creatures have “When this creature enters, destroy target creature with a lesser mana value.”

Constructs you control have haste.
When Urak, Voice of the Horde or another Construct you control enters, creatures you control get +1/+0 until end of turn.
Respawn (When this creature dies, if it attacked this turn, return it to its owner's hand.)

({T}: Add {B} or {R}.)
Urban Wastelands enters tapped.
Cycling {2} ({2}, Discard this card: Draw a card.)

{4}{B}{B}: Return Urchin Defector from your graveyard to the battlefield transformed.

When Urisk Fiddler enters, draw a card.

If you would create a token, you may instead create a tapped 2/2 green Faerie Bard creature token.
Whenever one or more creature tokens you control deal combat damage to a player, that player sacrifices an artifact or enchantment.

When Urn of Heroes enters, you may search your library for a legendary card, put that card into your graveyard, then shuffle.
{T}: Add {C}.

Haste
When Urud Stingswoop enters, it gains flying until end of turn.

Assault {1}{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.)
When Urudi Warlord assaults, it gains first strike until end of turn.

You may have Urushi, the Ripple in the Smoke enter as a copy of any creature on the battlefield, except it's an enchantment. (It's not a creature.)

Golden age (If you cast an instant or sorcery spell, activated a nonmana ability, and attacked this turn, enter a renaissance for this game.)
When Ushi Yun enters, create an Idol token.
As long as you're in a renaissance, artifacts you control have “{T}: Draw a card.”

Flash
Death knell — Put a +1/+1 counter on each creature you control. (As a creature you control dies, trigger one death knell of that creature or another permanent you control.)

Foretell {B} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)
Destroy target creature or planeswalker. You lose 4 life unless this spell was foretold.

Draw two cards, then you may foretell a card with foretell from your hand. It costs {0} to foretell this way. You lose 2 life. (You don't reveal that card. You can't foretell cards without foretell in this way.)

Foretell {X}{U} (During your turn, you may pay {2} and exile this card from your hand face down. Cast it on a later turn for its foretell cost.)
Gain control of target creature with power 2 or less. If this spell was foretold, gain control of target creature with power X or less instead.

First strike (This creature deals combat damage before creatures without first strike.)

Exile all lands. For each land card exiled this way, Uttermost Obliteration deals 1 damage to that land's controller.

Deception {1}{B} ({1}{B}, Return an unblocked attacker you control to hand: Put this card onto the battlefield from your hand tapped and attacking.)
Unblocked attacking creatures you control get +1/+0.

Whenever V-4 Fixit or another artifact you control enters, target creature you control gets +1/+1 until end of turn.

({T}: Add {B} or {R}.)
As Vacated Bar enters, you may pay {1}. If you don't, Vacated Bar enters tapped.

Melee (Whenever this creature attacks, it gets +1/+1 until end of turn for each opponent you attacked this combat.)
When Vaeldan's Liberator dies, you may exile it. If you do, return a permanent card with mana value less than or equal to Vaeldan's Liberator's power from your graveyard to your hand.

Other legendary creatures you control get +1/+1.
At the beginning of your upkeep, you may search your library for a legendary card, reveal that card, and put it into your hand. If you do, you skip your draw step this turn and shuffle.

As Vaertris' Master Plan enters, exile the top five cards of any player's library.
You may play cards exiled with Vaertris' Master Plan.
When there are no cards exiled with Vaertris' Master Plan, return it to its owner's hand.

[+1]: Until end of turn, whenever target creature deals combat damage to a player, that player mills two cards.
[–1]: Tap target creature. It can't become untapped for as long as you control Vaertris, Mindmelder.
[–6]: You get an emblem with “Whenever you draw a card, target player mills three cards” and “{2}{U}: Draw a card.”