Not every character leaves a corpse when they die. Certain undead individuals seem to disappear upon death - although it is unknown whether their bodies turn to ash, dissolve, or are spirited away to the void. In addition, the cause of death is significant - a character slain by a zombie or ghoul will immediately rise once more, while some angels will turn a victim to dust upon striking the killing blow.
A corpse is considered "fresh" if it is under 1 hour old. Any character can use the blood of a fresh corpse to write messages, although it is immoral to dishonor the dead in such a way. Defilers can use fresh corpses in their vile rituals, removing the "fresh" status in the process (and possibly causing the corpse to vanish in an unholy explosion).
Corpses last exactly 24 hours before vanishing from the Nexus, spirited away by the Graveyard Servitors to regions best left unknown. Sometimes, though, a Lich of sufficient power will compel the servitors to make a detour, delivering the old corpse to the Lich first.
Only player characters leave corpses; slain Pets do not.