msem-instigator

This spell costs {1} less to cast for each Zombie creature card in your graveyard.
When Undead Harbinger enters the battlefield, return target Zombie card from your graveyard to your hand.

You may have Undecided Mimic enter the battlefield 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.)

If you've scried this turn, this spell costs {2} less to cast and can't be countered.
Draw two cards.

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

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.”)

This spell costs {1} less to cast for each artifact you control. (This applies before Treasures are sacrificed for mana.)
When Underhanded Deal enters the battlefield, draw three cards.
Whenever you cast a spell, you lose 1 life.

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

As an additional cost to cast this spell, exile a creature card from your graveyard.
You draw three cards and you lose 3 life.

Enchant creature
Enchanted creature has first strike. (That creature deals combat damage before creatures without first strike.)

Undine Coven gets +1/+1 for each other Merfolk you control.
Whenever you cast a noncreature spell, Undine Coven deals 1 damage to target player or planeswalker.

Return up to two target creature cards with total mana value 4 or less from your graveyard to the battlefield.

Mill four cards, then return any number of creature cards with total mana value 4 or less from your graveyard to the battlefield.

Creatures you control get +2/+0 and have deathtouch as long as ten or more cards are in your graveyard.

Whenever a creature you control dies, you may return all cards that were inscribed onto that creature and all Auras that were attached to that creature to their owners' hands.

Unending Slime enters the battlefield with X +1/+1 counters on it.
{2}{G}{G}{G}: Return Unending Slime from your graveyard to your hand.

Flying
When Unfettered Atrocity enters the battlefield, destroy target creature or planeswalker.

Unfettered Noble can't be blocked unless defending player pays 3 life, discards a card, or sacrifices a nonland permanent for each creature they control that's blocking it.
Sacrifice a nonland permanent: Unfettered Noble gains indestructible until end of turn.

Exile target nonland permanent if its mana value is less than or equal to the number of creatures you control.
Lifecycle {3} ({3}, Discard this card: Create two tapped 1/1 colorless Elemental creature tokens.)

As an additional cost to cast this spell, exile X cards from your graveyard.
Create X tapped 2/2 black Zombie creature tokens.

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.

Destroy target creature with total power and toughness 8 or less.

Target creature can't block until end of turn. If that creature had a Steed attached to it, unattach that Steed and it can't block until end of turn.
Draw 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.