>>12821333Necrosis.
Some part of the body undergoes clinical death independently from the rest. Can be due to severe and prolonged constriction of blood flow, severe physical trauma, infection that the immune system could not defend against, or some combination.
Tissue decomposition spreads infection rapidly to surrounding tissue -- even when there was no infection to begin with, since rotten tissue is extremely unsanitary -- and surrounding tissue is effectively poisoned to death as well.
By this means the necrosis spreads from the initial site as if it were some kind of deep-tissue rash.
The most effective known treatment is to excise the dead and rotten tissue. If it's spread far enough, amputation may be the only way to do so.