Soul Binding

In theory, Soul Binding is the alchemical process of extracting a soul from a mortal body and infusing it into a vessel. Even in the most ideal circumstances this is believed to only be possible temporarily, as the empty body must remain alive for the soul not to be Sent (or turned into an Unsent). Additionally, it is highly likely that the vessel and soul will eventually reject one another as incompatible, causing untold damage to the soul in question.

That said, all of this is speculation-- the act of Soul Binding has been taboo since the beginnings of alchemy and remains forbidden to this day... everywhere except Zarthura, that is. In Zarthura, it is known that some of their highest-security prisoners in Oblivion are subjected to Soul Binding as part of their confinement. However, the methods by which they perform this procedure are a very well-kept secret.