This is one of the most dangerous spells in existence. Soul Transfers can only be performed by the Head Mage, and must be authorized by the Emperor. If performed right, it will allow the subject's body to swap souls with that of another—essentially, it is a body swap spell that allows for a few...modifications, depending on how the spell is performed and the level of the sorcerer. The subjects may or may not be able to access the memories and skills of their new bodies, or they could potentially lose their own if they so desire. To be honest, this spell has only been performed successfully a handful of times, so there is not much information about it recorded in the Tower's archives. However, if the spell-caster is too weak or makes a mistake, the potential consequences are devastating. One could lose their soul, having it shot out of their body into an abyss. Their soul could possibly be torn apart, either killing the person or erasing a chunk of their personality. Souls could also accidentally merge, forming one unstable soul with two conflicting minds of its own. Or the soul could be sent into the wrong body or, in one horrendous case, into that of an animal or inanimate object. Other side effects include madness, death, loss of speech and/or rational thought, narcolepsy, insomnia, constant nausea, vomiting blood, missing body parts, confusion, disintegration, mutism, loss of body function, etc. The success rate of performing this spell: 0.000000071%. The most common reason why people choose to perform this spell is to go into hiding and have the other stand in their place, usually as a sacrifice for an assassination. Other less common reasons include work, undercover operations, forbidden marriages, escaping punishment, depression, revenge, and fun.
It is generally not recommended that you attempt this at home, or ever. If you go up to the Head Mage demanding for a Soul Transfer, he will laugh in your face and turn you into a frog. If you ever acquire enough mana to get to the required level of power where you can perform this spell, and manage to find the incantation and all the other ingredients you need, you should try to find a bare place away from all living things before starting the spell. Also, make sure you write your will.