In my humble opinion, not enough games allow you to become Sauron, so I'd suggest a 'move essense' spell:

It enchants an item to contain your soul, and if your body dies the item compels a nearby monster to pick it up, after which you 'body jack' the monster, and can continue playing as this monster. Of course there have to be limitations to prevent this from basically becoming immortality, (which goes against the spirit of a roguelike, after all). And of course, this ought to be a more end-game ability, and rare. Perhaps require a rare book drop.