msem-instigator

Choose one —
• Unwrite deals 3 damage to target attacking or blocking creature.
• Destroy target artifact.
• Destroy target enchantment.

Ward {1}
Other Scouts you control get +1/+1.
Whenever three or more Scouts you control attack, draw a card.

Target creature gets -9/-0 until end of turn. Create an Idol token. (It's an artifact with “{1}, Sacrifice this artifact: Put a +1/+1 counter on target creature. Activate only as a sorcery.”)

Unzari Ruins enters tapped.
{T}: Add {C}.
Tap five untapped creatures you control: Unzari Ruins becomes a 10/10 Construct artifact creature with trample until end of turn. It's still a land.

Distribute two +1/+1 counters among one or two target creatures.

Rhapsody {4}{R}{W} (Pay {4}{R}{W} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
Lifelink, haste

At the beginning of each combat, harmonize twice. (To harmonize, trigger up to one harmony of a permanent you control.)
Harmony — Target creature gets +1/+1 and your choice of flying, haste, or vigilance until end of turn.

Flying (This creature can't be blocked except by creatures with flying or reach.)
Menagerie — When Uplifting Companion enters, if there are five or more creature types among creatures you control, up to two target creatures you control gain flying until end of turn.

Create a 2/2 white Angel creature token with flying.
Coalesce {6}{W}{W} (You may cast this from your graveyard for {6}{W}{W}. If you do, it enters as a Horror creature after it resolves. Exile it if it would leave the battlefield or not resolve.)
Flying

Whenever a creature dies, you may pay 1 life. If you do, draw a card.

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.

Menace

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.