something that follows..a demon?

I am having a hard time figuring out what is affecting my husband. Since he was a child he has had what he calls a “demon” that follows him and torments him. This “demon” usually presents its self when he talks about it but especially when things are going great and he is thriving; when he is happy. It has physically hurt him, shown its self to his sister and somehow, although he would scream, yell, etc., no one can hear him including the time with his sister and no one heard her either. I have seen him quit breathing for extended periods of time and writhe and wretch in pain, his nightmares are obscenely acted out loud, he is absolutely terrified. I know I am a strong person and because of that his “demon” tries harder to get to him it seems like.
Growing up my cousin would complain of like experiences but I never knew if I whole-heartedly believed him until I met my husband. Both my cousin and my husband turned to drugs when they were teens to try and escape the torture but it seemed that at times the altered state of mind allowed it to hurt them more. I suppose what I am getting at is; does anyone know anything about this? Has anyone heard of anything like this? Who would I talk to? A demonologist? Any tips, help or advice will be greatly appreciated!

As skeptical as I am about this sort of thing; and usually ascribe as a form of mental illness; this sounds serious and should be addressed by a professional, priest, exorcist, someone with experience in these matters.

There are a number of people on ATS that are much more versed in this subject than I and might be able to give more explicit instructions.

Good luck and best wishes to you and your husband.

I understand the skepticism and thank you, I really do appreciate ANY input

Could be manifestation of some childhood trauma. A good professional help provider i.e. psychologist or psychiatrist would be my 'first' course of action.
Good Luck