Maybe the human was part of an experiment gone wrong. Scientists in the fanfiction could have been conducting research in the underground facility, and something happened that made the human lose consciousness and then wake up there, surrounded by the remnants of the experiment.
One possible reason could be to gain extraordinary powers. In some fanfictions, giving up humanity might be a sacrifice for a greater magical ability, like being able to control all elements or access a new level of magic that is otherwise inaccessible to humans.
The human might find themselves in a strange and mysterious underground world filled with unique creatures. Maybe they'll encounter a group of underground dwellers who have their own culture and language, and they'll have to figure out how to communicate and survive among them.
One possible reason is that yoh might find the creativity in human fanfiction to be unoriginal. A lot of fanfiction tends to follow similar patterns or tropes, and yoh could be tired of seeing the same old ideas recycled over and over again.
One possible reason could be his excessive exposure to the arc reactor technology. The energy from the arc reactor might have gradually altered his DNA over time, causing him to become something other than human.
It could be just the result of someone's very strange and unique imagination. Some people like to explore extremely out - of - the - box ideas in their fictional creations.
Jealousy can also be a factor. Maybe Harry becomes jealous of Hermione's close friendship with another character, or vice versa. This could create tension and ultimately result in a break - up. In some fanfictions, Hermione's intelligence and success might overshadow Harry at times, causing him to feel insecure in the relationship.
One possible theme could be self - acceptance. As the character wakes up, they might have to come to terms with their own identity.
Magic is a common reason. For example, a cursed spell by an evil witch or wizard can cause the transformation. It could also be a result of a powerful artifact that has some sort of ancient magic within it.
Well, in fanfiction, there could be several reasons. It could be that Nico is suffering from some sort of after - effect of a battle. Maybe he was exposed to a harmful spell during combat and now his body is trying to expel the negative energy. Another possibility is that he's feeling homesick for the underworld in a way that his body rebels against the surface world, causing him to feel ill and throw up. Or perhaps he's caught a strange disease that is affecting demigods, and the first symptom is vomiting. And, of course, there could be some sort of emotional turmoil related to his relationship with Will that is so intense it makes him physically sick.
One possible reason could be a power struggle. Gerard might see Stiles as a threat to his own authority or plans within the story's context.