I did this vent a few months ago and had it lying around so I decided to upload it today!
I struggle with pretty violent IBS irl (I thankfully found some medication that helps with it, thank god) so sometimes it makes me wanna rip my fucking guts out.. its EXTREMELY unpleasant and has made me throw up from pain on multiple occasions, fun!
To cope with my irl problems I made up some lore for Jack, since I like making my poor man suffer as much as I do lmao here goes nothing:

Being half eldritch certainly has its pros and cons. Jack, as one of those beings for example, is stronger than the average human male, he can shapeshift and is pretty much immortal.
But, as with almost all things, those strengths come with great weaknesses too. One of the biggest drawbacks that he has to deal with because of his eldritch blood, is dealing with eldritch fever.

Definition:
Eldritch fever is a non-contagious, debilitating illness that flares up seemingly at random. The symptoms are similar to IBS in humans and are often so suddenly present that it is overwhelming and anxiety inducing. Infection occasionally occurs by bonding and mating with an eldritch being , in a sense you could consider it a sexually transmittable disease.

Symptoms:
Eldritch fever is a common and incredibly painful sickness that more than half of all hybrids and half-eldritch beings suffer from. It causes intense IBS and period-like stomach and bowel cramps. The whole process can be painful enough to cause the eldritch being to vomit from pain.
When going through a flare up, the being will often get suddenly overwhelmed by severe pain, nausea, dizziness, shaking limbs, an inability to breathe properly and a general weakness of the body.
During an episode of eldritch fever, the eldritch being will throw up an unnatural amount of the gooey, black slime that is usually found within their stomach, it is a powerful digestive fluid that is produced within the eldritch beings mucus layer that lines their stomach.
Flare ups can last anywhere from a few minutes to hours.
A lot of eldritch beings will get an intense urge to release their stomach contents, bowels and organs through any means necessary, some even going as far as gutting themselves or killing themselves in a desperate attempt to mitigate the pain.
Eldritch fever usually does not leave any long lasting damage, but tends to often take a toll on the eldritch beings mental state and wellbeing, as they are often living in fear from the next unexpected flare up.

Behavioral symptoms:
When going through a flare up, the affected eldritch being will often suffer from severe stress, anxiety and panic. They may enter a state of fight or flight where they desperately try to hide themselves away, most choosing to retreat into a safe space together with their soul mate. Some eldritch beings will lash out in anger at friends and enemies alike, their behavior closely resembling that of an injured and panicked animal.
Many eldritch hybrids suffer from extreme embarrassment and shame when confronted with their illness.

Causes:
It is uncertain why eldritch fever occurs in hybrids. The most plausible theory is, that frequent shapeshifting and movement of the internal organs puts a lot of stress on the still remaining human anatomy, causing intense upset in the stomach, an increase of slime and spasms/cramps within the intestines and organs.
Humans that have a previous history of IBS have increased risks of getting eldritch fever when soul mated.
Often times excessive stress will trigger and worsen the symptoms.

Treatment:
There is no known cure for eldritch fever, however the hybrids affected usually learn how to find remedies for their symptoms. Strong painkillers, drugs and tranquilizers will alleviate the symptoms slightly, but the only proven way to properly fight the fever is the presence of their soul bonded mate.
The bond between two eldritch being is incomprehensibly intimate and a soul bonded pair has the ability to heal one another in certain ways, for example through bodily contact and eldritch purring (a similar effect can be found in relationships between humans and cats, as their purrs are known to have healing properties). Soul mating is also often used as a form of therapy for eldritch fever, though it doesn't always has the desired effect since it can be hard to concentrate on mating when one of the parties is going through an intense flare up.