A group that wished to be 'ever-living' instead was cursed with 'never-dying'. Thier flesh rotted and fell fromthier bones but still they lived on. Now as skeletons they continue thier quest to remove thier curse. As skeletons they differ in that they do not need controlled or summoned. They are fully fledged NPCs with drives or ideas of thier own. Stabbing and slashing weapons would not affect them.
A certain culture believes that evil can live beyond the grave. They also believe that the destruction of the dead body can keep that one from coming back. Destroying the newly deceased body can destroy the soul if done properly. They first burn the body for 24 hours, smash the remains as well as possible, then burn the what is left for another 24 hours.