The Soul

Souls are the core of every living creature, and without one, a creature cannot truly live.

Transfer of Souls

Through certain dark rituals, a soul can be transferred from one body to another. It should be noted, however, that this type of magic is dangerous and can have catastrophic results.
For all intents and purposes, a "body" may not necessarily need to be that of a creature's. There have been a few instances of this technique being used to transfer a soul to constructs, or even certain objects.
Although the risks are large, a skilled caster could use this to gain a sort of pseudo-immortality.