On the principle of looking at everything: what is the Cotard Delusion?

The Cotard delusion or Cotard's syndrome or Walking Corpse Syndrome
is a rare neuropsychiatric disorder in which people hold a delusional belief that they 
are dead (either figuratively or literally), do not exist, are putrefying, 
or have lost their blood or internal organs. 

Young and Leafhead describe a modern-day case of Cotard delusion in a patient 

who suffered brain injury after a motorcycle accident:

[The patient's] symptoms occurred in the context of more general feelings                        of unreality and being dead. In January 1990, after his discharge from hospital

in Edinburgh, his mother took him to South Africa. He was convinced that he had been taken to hell (which was confirmed by the heat), and that he had died of septicaemia (which had been a risk early in his recovery), or perhaps from AIDS (he had read a story in The Scotsman about someone with AIDS who died from septicaemia), or from an overdose of a yellow fever injection. He thought he had "borrowed my mother's spirit to show me round hell", and that she was asleep in Scotland.

In rare instances, it can include delusions of immortality.