It is a couple of years old and i can't remember any names.
But I remember it starts out that somehow all of a sudden a human dies and regenerates extremely fast all of a sudden.
The society treats them as demons or aliens and quarantines them and experiments on them.
The protagonist i think is run over by a truck. (I might be completely wrong on this part) and finds out that he has become one of them.

These monsters basically can not die and are experimented upon.
Also on this part I am really hazy on but once they become essentially immortal they get a shadow that follows them and each one of them can see the other shadow. I don't remember what the shadows can do but the protagonist is special as i think he can control his better or it thinks on its own.

Any help will be much appreciated.

Ajin

Yes thank you that is it