msem-instigator

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

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.

Archive 0 (As this enters, shuffle the top zero cards of your main library into a new face up library. Sacrifice this after the archived library is empty.)
As Undue's Study enters, shuffle your graveyard into the archived library.

[+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.

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

When Undying Lich dies, create a 6/3 black Zombie Horror creature token with menace and “When this creature dies, create an 8/5 black Elder Demon Horror creature token with flying, deathtouch, and menace.”

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.

Create a 2/2 black Zombie creature token, then sacrifice a creature.

Return target creature card with mana value 3 or less from your graveyard to the battlefield.
Cycling {2} ({2}, Discard this card: Draw a card.)

Return up to two target creature cards from your graveyard to your hand. You lose 2 life.

Unearthed Legion's power and toughness are each equal to the number of cards in your graveyard.
At the beginning of your upkeep, mill a card.

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.

Create three 1/1 colorless Servo artifact creature tokens.
Exile Unending Armies. At the beginning of your upkeep, if you have more life than each opponent, you may cast it without paying its mana cost. Otherwise, put it into your graveyard.

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

Enchant creature you control
Enchanted creature gets +1/+1.
When enchanted creature dies, return it to the battlefield under your control, then arm it. (Create a colorless Equipment artifact token with “Equipped creature gets +1/+0” and equip {2}, then attach it to that creature.)

Target creature you control explores. Then it fights up to one target creature you don't control. If that creature has flying, the creature you control deals damage equal to its power to it instead. (Reveal the top card of your library. Put it into your hand if it's a land. Otherwise, put a +1/+1 counter on that creature, then you may put that card in your graveyard.)

Enchant land
When Unexplored Regions enters, draw a card.
Enchanted land is an artifact in addition to its other types and has “{T}: Add one mana of any color.”

Search your library for up to four basic land cards, reveal them, put them into your hand, then shuffle. You may play an additional land this turn.

Whenever one or more creatures you control deal combat damage to a player, that player discards a card. Then that player may discard another card. If they don't, create two 2/1 black Nightmare creature tokens with “This creature can't block.”
Fleeting {1}{B}{B} (You may cast this spell for its fleeting cost. If you do, sacrifice it at the beginning of the next end step.)

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

Search your library for a card, reveal it, then shuffle and put that card on top of it. If you revealed a noncreature card this way, each opponent loses 2 life and you gain 2 life.

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.)