Momonga would also face challenges in terms of adapting to the human lifestyle. He has to deal with things like finding shelter, getting food, and maintaining his health. In the game world, these aspects were not really relevant in the same way. He might not even know how to cook or find a proper place to live. And he would have to learn about money and how the human economy works, which is a whole new and complex system for him.
One challenge is communication. Dragons might not communicate in the same way as humans. So the human - turned - dragon has to figure out how to interact with both humans and other dragons. Another is dealing with the new instincts. Dragons have different survival instincts, like hoarding treasure or being territorial, which can conflict with the human's former way of life. They may also face threats from dragon hunters or those who see them as a threat to the existing order.
One challenge would be dealing with Spike's reputation. If other characters know Spike well, they might expect the reborn human in Spike's body to act in a certain way. The reborn human would have to constantly prove that they are different. Another challenge could be the physical cravings of a vampire. Since they have a human soul, they might struggle more with the urge to drink blood.
One major challenge is the physical pain. As the body changes from a human form to a dragon one, bones have to break and reshape, muscles have to grow and expand rapidly. This can be excruciatingly painful. For example, the elongation of the spine to support a long dragon tail would be like a searing pain throughout the back.
One challenge could be communication. Since lions communicate in different ways like roaring, growling and body language, it would be hard for a human to fully understand and be understood at first. For example, a simple warning growl might be misinterpreted by the human.
One challenge could be dealing with the change in how others perceive him. People who knew him as a boy might have a hard time adjusting, and he could face prejudice or misunderstandings. Another challenge might be adapting to the physical differences. For example, if there are differences in strength or body shape, it could affect his ability to do certain things like play Quidditch as he used to.
One interesting 'Momonga Turns Human' fanfiction could be the one where he tries to navigate the human world's social norms. He has to learn basic things like making friends, which is a huge change from his undead life in the game world.
He might face social challenges. For example, people may treat him differently based on his new appearance as a girl. He could also face challenges in relationships. His old friends might not know how to act around him anymore.
Survival would be the main challenge. Lack of breathable air is a big issue. The human would need to figure out if the planet's atmosphere is suitable or find a way to create or purify air. For example, if there are certain plants on the planet, they might be able to use them to produce oxygen somehow.
One big challenge for Benhypno in human form would be dealing with his memories and instincts from his alien self. He might have flashbacks or urges to act in an alien way when he's in a tense situation. For example, if he's in a fight, his first instinct might be to use his full alien powers, which he can't do without revealing himself. Also, he may face the challenge of making friends. Since he has a different background, he may find it hard to relate to normal human teenagers and their interests.
They could face hostile monsters, difficult puzzles, and the struggle to find a way back to the surface.