msem-instigator

Menace
Whenever an opponent discards a card, monstrosity 2. (If this creature isn't monstrous, put two +1/+1 counters on it and it becomes monstrous.)

Reach, deathtouch

Whenever Twilight Dancer attacks, harmonize. (Trigger up to one harmony of a permanent you control.)
Harmony — Each player loses 1 life and mills a card.

Gestalt (When this or another creature enters under your control, you may merge it with the one in play, keeping counters and attachments. They become the permanent on top plus all abilities from under it.)
Lifelink

Double strike
Ride {2}{W} ({2}{W}: Transform this and attach it to target creature you control without a Steed. Ride only as a sorcery.)

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)
{1}{B}, Sacrifice a creature: Put a +1/+1 counter on Twisted Cultist.

Distorted (This creature can be blocked by tapped creatures but not untapped creatures.)
Revive {1}{B} ({1}{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.)

{3}{U}: Tap target creature. Activate only as a sorcery.
Cycling {2/U} ({2/U}, Discard this card: Draw a card.)

Golden age (If you cast an instant or sorcery spell, activated a nonmana ability, and attacked this turn, enter a renaissance for this game.)
Whenever a Demon you control enters, put a +1/+1 counter on Tzelin.
{2}{B}: Sacrifice Tzelin. If you do, each opponent loses life equal to Tzelin's power. If you're in a renaissance, also draw that many cards.

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 Windrider envisions an instant or sorcery card, it gets +1/+0 and gains flying until end of turn.

Unassuming Waif can't be blocked.
Unassuming Waif gets +1/+0 for each other Warrior you control.

Encourage ({T}: Another target creature you control gets +1/+1 until end of turn. Encourage only as a sorcery.)
Whenever a creature deals combat damage to a player, if it was encouraged by Unbeholden Navigator this turn, draw a card.

{T}, Pay 1 life: Add one mana of any color that a land an opponent controls could produce.

Colossal 2—{4}{B} (You may cast this for its colossal cost to have it enter with two +1/+1 counters; or pay {4}{B} as a sorcery: Put two +1/+1 counters on it if it has none.)
Lifelink

Menace
{3}{B}, Discard a card: Unhallowed Singers gets +3/+0 until end of turn. Whenever it deals combat damage to a player this turn, they discard a card.

{T}: Add one mana of any color.
{T}, Sacrifice Unicorn of the Vale: Add three mana of any one color.

Whenever you cast a noncreature spell, create a 1/1 colorless Construct artifact creature token.

{G/W}: If Unlikeliest Heroes doesn't have a +1/+1 counter on it, put a +1/+1 counter on it.
{2}{G/W}{G/W}: If Unlikeliest Heroes has a +1/+1 counter on it and doesn't have vigilance, put two +1/+1 counters and a vigilance counter on it.
{3}{G/W}{G/W}{G/W}: If Unlikeliest Heroes has vigilance and not indestructible, put four +1/+1 counters and an indestructible counter on it.

{6}{W}: Tap all creatures target player controls.

Discard a card: Unseen Maniac gets +1/+1 and can't be blocked this turn.
Whenever Unseen Maniac deals combat damage to a player, draw a card.

Rhapsody {4}{U}{U} (Pay {4}{U}{U} to cast this as a 4/4, or as a sorcery to make it become a 4/4.)
When Unstable Prophet enters, surveil X, where X is its power. (Look at the top X cards of your library, then put any number of them into your graveyard and the rest on top of your library in any order.)

{T}: Target attacking or blocking creature gets +1/+1 until end of turn.

Menace (This creature can't be blocked except by two or more creatures.)
Engage (This creature must be blocked each combat if able.)

When Unwitting Fanatic enters, create a 3/2 colorless Horror creature token.
Sacrifice Unwitting Fanatic: Another target permanent you control gains hexproof and indestructible until end of turn.

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