Never heard of this before until I was reading something. Quite a bizarre mental illness...

Cotard delusion is a rare mental illness in which the affected person holds the delusional belief that they are already dead, do not exist, are putrefying, or have lost their blood or internal organs.[1] Statistical analysis of a hundred-patient cohort indicates that the denial of self-existence is a symptom present in 69% of the cases of Cotard's syndrome; yet, paradoxically, 55% of the patients present delusions of immortality.[2]

Continued...

https://en.wikipedia.org/wiki/Cotard_delusion