msem-instigator

Tzelin of the Demon Realm can't block.
Whenever a Demon enters the battlefield under your control, you may pay {B}. If you do, return Tzelin from your graveyard to the battlefield with a +1/+1 counter on it.

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 enters the battlefield under your control, 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.

You may play cards exiled with Azamir planeswalkers.
[+1]: Target player exiles a card from their hand.
[–3]: Exile target nonland permanent unless its controller exiles two other permanents they control.
[–7]: Target opponent may have you take two extra turns after this one. If they don't, you control them during their next turn.

Equipped creature gets +2/+2.
Whenever equipped creature attacks, creature spells you cast cost {2} less to cast this turn.
Equip {2}

Whenever Ujuraq, Elder Watcher enters the battlefield or attacks, surveil 1. (Look at the top card of your library. You may put that card into your graveyard.)
Ujuraq gets +3/+3 and has trample as long as there are eight or more cards in your graveyard.

This spell costs {1} less to cast for each Scarecrow you control.
Other creatures you control get +0/+2 and have vigilance.

Other Hounds you control have trample.
{R/W}, {T}: Put a +1/+1 counter on target Hound you control.

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

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

[+2]: Exile up to three target cards from graveyards.
[+1]: Create a 2/1 black Horse Zombie creature token with lifelink.
[0]: If Undue has six or more loyalty counters on him, each opponent returns a creature card of their choice from a graveyard to the battlefield under your control.

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

If you would draw a card, you may return an instant or sorcery card from your graveyard to your hand instead.
{U}: Draw three cards.

When Unity Spheres enters the battlefield, create two 1/1 colorless Construct artifact creature tokens.
Tap two untapped creatures you control: Add two mana of any one color.

When Unmarked Vials enters, create a Vial token. (It's an artifact with “{1}, Sacrifice this and another permanent: Draw a card. Activate only during your turn.”)
{1}, {T}: Add one mana of any color.

Glitch—Sacrifice a creature. (Remove one [restriction] for each time you pay the glitch cost.)
When Unshackled Revolutionary enters the battlefield, you may destroy target creature an opponent controls [with mana value 3 or less]. [You lose 3 life.]

Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)
Unstable Borer attacks each combat if able.

When Unstable Orb enters the battlefield, draw a card.
{3}, {T}, Sacrifice Unstable Orb: It deals 2 damage to target creature.

Constructs you control have haste.
When Urak, Voice of the Horde or another Construct enters the battlefield under your control, 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.)

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

You may have Urushi, the Ripple in the Smoke enter the battlefield 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 the battlefield, create an Idol token.
As long as you're in a renaissance, artifacts you control have “{T}: Draw a card.”

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

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.

[+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.”

As long as an opponent has one or fewer cards in hand, Valdez gets +2/+0 and has menace.