Cauldron of Souls

{T}: Choose any number of target creatures. Each of those creatures gains persist until end of turn. (When it dies, if it had no -1/-1 counters on it, return it to the battlefield under its owner's control with a -1/-1 counter on it.)

Wick, the Whorled Mind

Whenever Wick or another Rat you control enters, create a 1/1 black Snail creature token if you don't control a Snail. Otherwise, put a +1/+1 counter on a Snail you control.
{U}{B}{R}, Sacrifice a Snail: Wick deals damage equal to the sacrificed creature's power to each opponent. Then draw cards equal to the sacrificed creature's power.

Necrotic Plague

Enchant creature
Enchanted creature has "At the beginning of your upkeep, sacrifice this creature."
When enchanted creature dies, its controller chooses target creature one of their opponents controls. Return this card from its owner's graveyard to the battlefield attached to that creature.

Endemic Plague

As an additional cost to cast this spell, sacrifice a creature.
Destroy all creatures that share a creature type with the sacrificed creature. They can't be regenerated.

Pestilence

At the beginning of the end step, if no creatures are on the battlefield, sacrifice this enchantment.
{B}: This enchantment deals 1 damage to each creature and each player.

Vile Consumption

All creatures have "At the beginning of your upkeep, sacrifice this creature unless you pay 1 life."

Exotic Disease

Domain — Target player loses X life and you gain X life, where X is the number of basic land types among lands you control.

Smallpox

Each player loses 1 life, discards a card, sacrifices a creature of their choice, then sacrifices a land of their choice.

Traveling Plague

Enchant creature
At the beginning of each upkeep, put a plague counter on this Aura.
Enchanted creature gets -1/-1 for each plague counter on this Aura.
When enchanted creature leaves the battlefield, that creature's controller returns this Aura from its owner's graveyard to the battlefield.

Each player loses a third of their life, then discards a third of the cards in their hand, then sacrifices a third of the creatures they control of their choice, then sacrifices a third of the lands they control of their choice. Round up each time.

Wound Reflection

At the beginning of each end step, each opponent loses life equal to the life they lost this turn. (Damage causes loss of life.)

Illness in the Ranks

Skullcap Snail

When this creature enters, target opponent exiles a card from their hand.

Spreading Plague

Whenever a creature enters, destroy all other creatures that share a color with it. They can't be regenerated.