msem-instigator

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

Target creature gets -X/-X until end of turn, where X is 2 plus the number of cards named Undead Servant in your graveyard.
As long as this card is in your graveyard, its name is Undead Servant.

When Undead Necromancer enters, search your library for a card, put that card into your graveyard, then shuffle.
As long as this card is in your graveyard, its name is Undead Servant.

When Undead Servant enters, create a 2/2 black Zombie creature token for each card named Undead Servant in your graveyard.

You may have Undecided Mimic enter 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.)

Enchant land you control
Whenever enchanted land is tapped for mana, it produces an additional mana of that type.
{1}{G}, Sacrifice Under Untamed Skies: Destroy target creature with flying.

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.”)

Slipstream (This creature can't be blocked as long as only creatures with slipstream are attacking.)

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

Up to one target creature gets -2/-2 until end of turn. Put a +1/+1 counter on up to one target creature you control.

When Undermarket Favors enters, target player discards a card, then mills four cards.
{1}, Sacrifice Undermarket Favors: Draw a card.

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

Bleed {1}{B} (You may cast this spell for its bleed cost if an opponent lost life this turn.)
When Undermurk Scavenger enters, you draw a card and you lose 1 life.

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

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

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.

Zombiecycling {1}{B} ({1}{B}, Discard this card: Search your library for a Zombie card, reveal it, and put it into your hand, then shuffle.)
Unearth {1}{B} ({1}{B}: Return this card from your graveyard to the battlefield. It gains haste. Exile it at the beginning of the end step or if it would leave the battlefield. Unearth only as a sorcery.)

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.

When Undying Watcher enters, germinate. (Put a +1/+1 counter on a creature or create a 1/1 green Saproling creature token.)
Sacrifice another creature: Undying Watcher gains indestructible until end of turn.