Manipulation and Transformation

Of the three basic rules to the Cycle of the Soul, during one's life it is possible to manipulate the soul in a variety of forms, either by adding or removing energies from the soul, or by separating the components of the soul.
In the realm of adding and removing life or death energy, the magic is almost solely practiced by Life and Death, although a few methods of more crude methods have been spread by apprentices1 over time. Overall, the practice can be generalized into a few categories, provided that the recipient is an angel or otherwise traditional2.
Through the addition of life energy or the removal of death energy (rare), lifespan can be increased, or internal bodily dysfunction may be remedied through a combination with other medicinal treatment. Though some historical students of Life's practiced this in its weaker forms, generally this magic is limited to those who choose to make Deals with Life or Death, as the amount of energy required to seriously affect lifespan would drain anyone else attempting to do so. As even the weaker forms still slowly weaken the practitioner over time, those who take on the study often only use it in cases of dire need, where easier medicinal treatment would not work with enough speed; even this is generally limited to heads of state3.
Next, the addition of death energy causes the quickening of aging of the soul (though it will generally have no immediate effect on the body). For obvious reasons, this is usually reserved for the payment to Death for a deal where the offered service is something else (generally knowledge). The only other occasion when this magic4 is used is by Life's apprentices who are in the service of demonic heads of state.
Thirdly, the removal of life energy in near all cases of its use causes an instant, relatively painless death; solely used by the most desperate. Usually paired with trying to save someone else's life either by transferring the energy (if done by Life) or by removing death energy (if done by Death). Sometimes, deals are made wherein one's life will be traded for killing someone else, although these are rare and generally done by those who are nearing dying.
Beside these, the realm of dividing the soul is solely one of rituals, passed down from the original deals between the clans and Life and Death. During Elemental Transformation, the components of the soul are forcefully pulled apart, one half completely removed from its bond with the rest of the soul and body. By doing so, the decay of the soul is halted, and through the methods of the physical component of the ritual, the body is similarly suspended in age and state. Once this magic has been performed, the effects are almost always permanent5. For light and water angels, this means that the only part of their soul still bound to their body is their life energy, while for fire and shadow demons, this is implied for their death energy.
There is, in theory and (singular) practice, a method to binding more than soul to the body, in a sense "multiplying" how many are present, however this is only ever in the context of the souls of sentient creatures, where the souls do not merge and would instead remain distinct6.

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1 "apprentices": Life's apprentices only, though the magic practiced sometimes deals with death energy. Death does not train doctors.
2 "angel or otherwise traditional": While angels share most living things in relying on the life energy left in their soul, and thus are affected as humans, the effects are reversed for demons. For example, adding death energy to a shadow demon's soul will act as a healing agent.
3 "limited to heads of state": far in the past, Life was less stringent with her requirements for becoming her student, and the magic was much more popularly practiced. For the past few centuries, however, there have been very few whom Life have trained, and for this reason their services are often revered and only afforded using on the crown. As always, there are exceptions, as a noted few have chosen instead to use the magic for their personal purposes on loved ones, but most enjoy the status that comes with their training and working for a crown. For this reason, particularly few hail from Avalon, and bizarrely, this means that Life's least preferred Darvaz is often where they choose to take their services, happily accepting the emperor's luxuries.
4 "this magic": despite being a similar concept, Life does not often train demons, and for this reason the technique is slightly modified, but the effect is the same so the difference is not great enough to warrant its own category.
5 "almost always permanent": occasionally, the form of payment in deals involves reintroducing the missing energy to the soul, restarting the aging process. Abayan performed a good example of this type of deal.
6 "the souls...remain distinct": as personality and character are tied to the sentient soul, this would cause a sort of literal multiple personalities effect, wherein the souls would at worst battle for control, inevitably forcing themselves of out mutual misery by destroying the body, or at best neutralize each other in internal conflict, turning the person into a self-stopping husk. If by some miracle one soul overpowered the other or they were in consensus, it would be possible for the body to be of extra strength and reinforcement, as is with Death's continued existence, but there always remains the chance of the dominant personality losing control.