Cotard's syndrome is one of the rarest mental disorders it is characterised by body part negations ( ie beliefs that body part are missing rotten...) the person my also believe they are dead/ immortal and damned.

image of the gyrus fusiformis:
https://images.app.goo.gl/RpEULyKUmHJ...

Royalty free picture used in the thumbnail:
https://www.istockphoto.com/en/vector...