The protagonist is an ordinary guy. He goes to school, then he works, then he studys, and then he relaxes.

Yup nothing strange.

Except maybe one thing.

Since he was five he'd have night terrors every night where he would be tortured by a blob of grey goo attached to a strange white mask.

Shortly after he turned seventeen he made a deal with the mask.

It turned him into a demon and now he has to live an ordinary life despite the many changes to his ordinary life.