Their back up characters rescue their bodies and have them rezzed at the local temple (or reincarnated). In old school D&D, players should probably have more than one character, plus assorted henchmen. Not all of whom go on the same adventures.
For instance, EGG has Mordenkainen, Yrag, later Bigby (apparently originally a henchman)
Or you could have a lich or something do it, then geas them to do something nasty.