Yokai Blight

Eons ago, the Great Demon Tamamo created a curse in order to weaken the followers of Kumiho. The plague expanded dramatically, corrupting and transforming mortals into monsters.
Later, after Tamamo gained followers herself, she created stones which encased some of her own powers, later named Yokai Blight. The battles that followed damaged the land but mostly the people living in it. Monsters started to spawn everywhere, killing without mercy in uncontrollable frenzy.
They were later known as Oni, mortals who were converted and twisted by demonic powers. After loosing the war they were banished to the Abyss, the plague with them.
But now that Tamamo as awoken, the Yokai Blight was reintroduced in a world who forgot about it, and now as to deal with the consequence of their ignorance.

Transmission & Vectors

The transmission is either a ritual by an agent of Tamamo, or by voluntary exposure and use of the Sassho-seki stones. It's not infectious, someone can't transmit it by bite or blood. But the Blight can be "gifted" to someone who wants it.

Causes

The cause is mystical. As the Great Demon Tamamo was destroyed centuries ago, her body was send back to the Abyss, breaking in the process. The Sassho-seki were created from the pieces of her life-force, and cultivated by the Oni who were also banished from the Prime Material.
Now that the Oni have infiltrated the people of the surface again, they brought with them the now fully powered stones, and started experiment or just letting them in the world for the unlucky ones who would take them.

Symptoms

It depend of the method of infection and what is infected, the effects being different for monsters or players. The effects on players are described in the Sassho-seki Stone page.
The monsters have the following rules :
A monsters infected by the Yokai Blight gain Xd10 Maximum hit points, X being half it's CR rounded down. If you are not using the CR system, you can give them a total of dice equal to half the average character's level rounded down. They can gain any number of the following Yokai capacities :
Yokai Waters
Each time a monster attacks, whether it hits or not, it can drop a 5ft radius pool of Yokai Waters centered on the target of the attack. A player character beginning or finishing it's turn inside a pool must make a DC 13 Constitution Saving Throw. On a fail, it takes necrotic damage following the damage table below and has it's movement speed reduced by half until the end of it's next turn. On a success, it takes half damage and doesn't have it's movement speed reduced. Their can't be more than one pool centered in the same place, but they can overlap. In case of a player character standing inside two or more pools that overlap, it suffers the effect only one time.
The damages scale with the levels of the player characters as follow :
Levels/Damage : 1-5/1d4+2 6-10/2d4+4 11-15/3d4+6 16-20/4d4+8

Yokai Frenzy
When a monster has it's Hit points reduced to 0, it become immune to all status conditions, all damage types and have it's movement speed multiplied by 4. It acts directly as a legendary action and use all it's movement to go towards the player character who killed it and make a melee attack against them with the following rules :

The monster make 3 attacks as it's multi-attack. If it already possessed this action, it's replaced by this one.

The attacks use the first melee weapon on the monster sheet. If it doesn't have any weapons, it makes an unarmed weapon attack dealing 3+ Strength modifier necrotic damage.

Attacks have a +2 to hit and damage, and become necrotic instead of their normal type.

Once this multi-attack is finished, the monster dies and it's body start to decay instantly. Any investigation check must be made within 5 in game minutes or be destroyed except for the magic items.
In addition, you can make a monster who is "Yokai Born". It follows the same rules as above but wasn't exposed to a Sassho-seki. It's the only instance that the Yokai Blight can be transmitted to other things, if the monster choose to do so.

Treatment

The Keiryu Order could be able to "cure" the Blight with some spells or rituals, but their is nothing sure since it's the first time in eons that the Yokai Blight has been released. Death is for now the only sure way of dealing with it, since the Blight doesn't spread by itself.

Affected Groups

Anything can be affected by the Yokai Blight, as long as the victims are exposed long enough to it, on purpose or not.

Hosts & Carriers

The Yokai Blight is more some kind of curse but understood as a disease since nobody was aware it was a thing until now. It's carried places to places by mystic stone known as "Sassho-Seki". But some creatures or people are "Yokai born", which mean they didn't need exposure, they were created from the Blight itself at some point by a powerful entity.

History

The first case of the "disease" is meant to be discovered by the players. The outbreaks and the like are at the discretion of the Dungeon Master, since anything alive can be exposed and affected.
The Keiryu Order is the only organisation that knows what the Yokai Blight is, since they keep it secret inside the Order and martially and mystically train their monks and priestesses to deal with it (specifically by being able to destroy the Sassho-seki stone that carries the Blight without being affected)

Type

Divine

Origin

Divine

Cycle

Chronic, Acquired

Rarity

Extremely Rare