less of a horror, but close, horror-scifi possibility. the cancer guy, he was made almost entirely of cancer cells, cancer like cells which allows him to have rengenerative powers, and can regenerate a whole body by forming a new one, and eats tumors from medical waste to survive, and can detect cancer in other people using his ability to emit x-rays, cat scan/pet scans(he was able to detect sullys tumor after they removed the alien tracking chip from her.) sorta like resurrection that cylon uses, his conscioness is transferred to the new body. the premise was he was stalking and people who had tumors(they may not be aware of ) and killing them and cutting out the tumors.

hes probably the strongest villian in the show, he couldnt be killed because somehow he use magic to ressurect himself, or made "justsu clone" of himself, hence his appearance in a random airport in india. i think you search clips of it on youtube.

and i dont remember what happened , with the boy he was STALKING mysteriously why shapeshifted into a janitor/his mom, also the ability to teleport.