Kiseijuu, Jagaaaaaan Recommendations
In both manga's the main character obtains power from a strange organism. This power would usually turn the character into a monster of sorts, however our protagonist is an exception and fights these monsters. Along the way he meets allies and enemies alike. Both manga is packed full of action and is an interesting read.
Both manga feature some kind of "monster" or "alien", and they take over people's bodies.
In Jagaaaaaan, there are frogs that rain from the sky, then take over emotionally vulnerable people, and the main character isn't fully taken over.
In Parasyte, There are Parasytes that fell from the sky, and they all took over the body of the nearest human. The main character stopped the parasyte from entering his brain, and it takes over his arm.
Are both dark fantasy
Both main characters is some how involved with the unknown species
Both humanity under attack by allein or monster
Both are pretty gory.
Both have enemies that eat humans for no reason.
Both about humans gets infected by something.
I think if you enjoyed one, you would enjoy the other.
It was directly inspired by Kiseijuu and referenced in the story itself. The Parasytes are represented by frogs in this case and the posses the body of the victim making them a fractured human. Although unlike kiseiju, jagaaaaaan's fractured humans are possessed by their dominant primal urges.
Art: Jagaaaaaan has a modern experimental-esque art while kiseiju art is a little dated but still great.