The MC could become a penguin by accidentally entering a forbidden area or getting involved with绁炵 powers. Challenges? Well, there could be problems like not being able to move fast on land or struggling to fit in with the penguin community.
A mentor figure could be the one betraying the mc. The mc trusted this mentor completely, but the mentor has some hidden motives. Maybe they are working for a greater evil and see the mc as a threat or a tool to be used, so they betray the mc, which then causes the mc to turn dark.
I'm not sure specifically as there could be many 'evil mc' (evil main character) light novels out there. It might be about a protagonist with a dark or villainous nature and their adventures, conflicts, and growth in a fictional world.
One possible plot could be that Sakura is constantly overlooked and underestimated in her world. She gets tired of being in the shadow of others and decides to use her skills for her own gain in a more self - serving and unethical way. For example, she might start using her medical knowledge not to heal but to manipulate people's health for power.
One possibility is that Danny gets tired of constantly being the hero and facing so much pressure. He might start to think that if he was on the other side, he wouldn't have to deal with all the expectations. So he begins using his powers for his own gain, like scaring people to get money or to make others do what he wants.
I'm not entirely sure specifically about this 'evil ash betrayed fanfiction' as there could be many different interpretations. It might involve the character Evil Ash in a situation where he betrays someone or something within the context of the fan - created story.