Blood Legacy: New World of Doom - Chapter 123: Volunteer Donors

Thrown into his favorite novel as a powerless extra, he never expected to survive, let alone tame the gods.In a world where strength is everything, chaos reigns and beasts roam the land, including fearsome dragons and gods, the stakes have never been higher.With the power to tame beasts that even gods fear, he must rise above a world on the brink of collapseBut how will he fight for his place in a doomed reality!

A teenager named Evan Mason was reincarnated into the magical world of Harry Potter and goes to school at Hogwarts!
With only his knowledge of the future and great talent for magic, he takes the path towards the top of the magic world, only to realize that ever step he took, changed everything!

- Description from Novelupdates

The world I transmigrated into was a dark fantasy game universe, devoid of hope or dreams.Doesn’t feel like it at all, though.

The story is about a man whose compulsive tendencies make it impossible for him to keep a job, and a young girl who skips school and loves insects. The two meet, come to support each other in reintegrating into society, and fall in love. There’s a problem, however: namely, the parasites in their heads.