Unconventional uses of old technology (or in this case, sorcery), can work wonders. A desperate but clever necromancer made up a way to turn just one body part undead. When you are bleeding out from a ruptured stomach, you tend to try and solve it, and this one succeded.
Through the use of twisted energy reversers and the creation of a neutral parasite entity, a ruined or dying body part can be revived as undead. Though reversing energy is core to necromancy, leaving a continous converter inside a living body (other than as an attack) was quite unheard of. Apparently, it worked.
There’s good things with undead body parts (they are pretty damn durable), but you might also get sick, experience massive necrocis, slowly turn into a zombie, or what have you. But at least it’s better than dying. You might even be able to fix it later (get a cleric to ressurect your dead lungs!).