Hellboy AU
Master Fu is a specialist and scientist who studies the super natural. As a younger man, he is on an expedition in China investegating magical occurrences. During his search he witnesses a bizarre event. Two beings appear from a portal that leads to another world. One is the powerful demon and keeper of hell Hawkmoth and the other is his wife the good witch and keeper of the stones of destruction and creation Le Peon. Hawkmoth reveals that he has seen the evil in humanity and wants to unleash hell on earth to destroy it using the stone of destruction then recreate it into new and better world using the stone of creation. Le Peon being a kind soul and a human herself disagrees and tries in vain to convince her husband to forget his plan. When he refuses and demands she give him the stones she has no choice but to use all her magic to send her husband back through the portal and close it.
However using up her magic like that left her body very weak, Fu goes to help her and discovers she's pregnant. The shock of the event causes her to go into labor and due to her weak body she dies shortly after giving birth to a half demon son with cat ears, a tail, claws, and cat eyes. Before dying she tells Fu that her son's name is Cat Noir and she warns him that one day her husband will return for the boy because she swallowed the stone of destruction during her pregnancy resulting in the child being born with the power of destruction. When Fu asks where she hid the stone of creation she tells him that she hid it in the belly of a human woman she did not know. (She was in a rush) With her final breath she begs him to protect her son.
Fu having always wanted a family adopts the child and renames him Adrien. Adrien is raised in a secret government facility where paranormal beings are kept hidden and protected while agents are trained to hunt paranormal dangers. Among the beings kept there are Carapace (renamed Nino) a mutant turtle boy, Alya an alien girl from another dimension, and seven imps fprmed from rock Tikki, Plagg, Nooroo, Pollen, Duusu, and Trixx. As a child Adrien is a sweet boy who loves cats and wants to go outside but can't due to his strange appearance.
When Adrien, Nino, and Alya are still children they get a new a member to the facility. A young human girl named Marinette born with the ability to give off a powerful light that can create and heal but she has difficulty controlling it and when she feels threatened she triggers a powerful blast that protects her by burning anything it touches. Due to accident she caused her parents were forced to turn her over to the government. When she arrives she's scared, confused, and refuses to speak to anyone. Adrien is instantly smitten with the girl and gets her to open up by making her laugh and giving her flowers. The four of them become friends.
Years later now adults, Adrien, Nino, and Alya have become trained hunters who eliminate super natural dangers to mankind while Marinette had left the organization to seek out her parents and try to live a normal life. Adrien having spent his whole life in the organization, never allowed to leave except for a mission has grown restless to get out not to mention he is deeply in love with Marinette and misses her so he sneaks out to see her which Fu, Alya, and Nino advise against. Meanwhile a cult of devil worshipers while performing a ceremony accidentally open a patrol that releases Hawkmoth who after learning of his wife's death is now more determined than ever to unleash hell on earth. With help up devil worshipers and revived, undead assassin he seeks out Adrien and the one born with the power of creation who unknown to all is Marinette. Her mother was the woman who's belly Le Peon hid the stone of creation in and also like Le Peon she was pregnant. (But it was during the early stages) Resulting in Marinette having her powers. BTW the reason Marinette doesn't look like a ladybug hybrid is because she was born from human parents, Adrien was born from a demon father and human mother. What do you think? Comment if you want me to write a fanfic off of this.