Corpses in the Valley of the Dead, or at least corpses created in the Valley of the Dead, should be preserved indefinitely without rotting away. This could be implemented either by simply keeping them fresh and unaging, or they could get completely rotten and bad to eat, halting just before they would have rotted away completely.

Not specified if this would have an effect on wraith corpses from the Valley’s graveyards, which instantly rot away if they produce a “Blecch!” message.