The body of a character who permanently died will decay into a Skeleton after 24 hours.
Descendants of a deceased character can bury these remains to be able to regain lost proficiencies. See the Burial Rights skill to learn more about this.
Sources of Death
Permanent death is either the result of Disease, Murder (either by players or darkness creatures), or getting KO during a Blood Moon. On expedition servers it's also possible to be permanently killed from Cannon shots.