Necromancers have always been able to order around undead armies and generally bend entities of negative energy to their whims. But why settle for undeath by proxy? D12 hit dice and a list of immunities longer than your arm sounds like a good time for any spellcaster, but of course, trading in your meat bag for another, more gangrenous meatbag with an abysmal Fortitude save has its own disadvantages. Really, there's no reason to trifle with bodies at all - once you've crossed that hurdle into undeath, a mid-level spellcaster can make themselves a haunting presence, and go around possessing one item, Poltergeist-style. Of course, you're not just going to pick any Tom, Dick or grand piano off the street: if you get to possess an object and pilot it around, you might as well custom-make the best you can get.
