They wake up as undead. Now that the afterlife is lost to them [3e Manual of the planes], they must find a way to have thier spirits restored [ ressurection is the lowest level spell that can do this ] so they can rest properly or face black oblivion upon destruction in the mortal world. Or just eat people if thats what they would rather do.
Let the players play the monsters that killed them, D&D seems to be more about the items than the characters, so this could prove amusing as the critters 'suit up' and strike back against the [demi]human settlements that sent the 'assasins' after them.