From the outset I would like to note that this is only a theory and that there is a very large possibility that full necromantic transfiguration is not even possible. It is called a theory and continually retains the title of theory because it is only a theory (!). Any attempt to consider this an absolute claim or even to necessarily say it is possible to hold a perpetual transfigured state will be fallacious from the beginning.

Necromancy is commonly thought of as a dangerous art to those who fall under its grasp but also to those that attempt to use it since, unless one is an expert at self-deception, none will claim to be able to shed their primal desires while bound to the mortal flesh; necessarily, necromancy poses a certain threat to the longevity of the practitioner in certain circumstances where the power is being used. The practitioner is also at risk of losing health by the use of necrotic energies and communing with geists on a regular basis since a necessary requirement for communication with these entities is that the life of the living be reduce and the life of the dead be increased such that the two entities find a common state to understand one another. Communication with the dead requires the necromancer to stand astride the rift between life and death and, in some ways, become the half-dead themselves so that the living and dead can effectively transfer information between the two realms. Regardless of how one may approach the subject or sub-divide or rationalize the practice the indisputable fact is that the necromancer is a living being who is coming into contact with the powers of death. As such, it is often considered a simply undeniable occurrence that the necromancer's own health begins to slide as a result of their practice; this trend is reflect in almost every written record where necromancy has been practiced. This is seen in historic accounts of necromancy where the necromancer has charmed a ghost into prophesying the future but after the initial prophecy has been received the ghost then continues to inform the necromancer of their own imminent death, and also there are some cases where the necromancer has reported overtly that the sight of death has chilled them to the bone and congealed their blood (thinking specifically of Orpheus and comments in Shakespearean drama concerning the appearance of ghosts). Touching death is considered a taboo in many cultures, both figuratively and literally. Levitical laws prevented those who have touched the dead or associated at length with those things of death from entering the common camp for a specified period of time to ensure that they were not blighted (though this is obviously simply a bacteriological consideration) and there are even myths in some shamanic legends that seeing a ghost means that one is destined to die shortly thereafter. From antiquity to the present, necromancers have earned the stereotype of being atrophied, unkempt (such as the Aristophanes' portrayal of Chaerephon), and prone to the life of a hermit due to their fascination with death. These character traits are attributed to the withering power of touching metaphysical death.

The theory of necromantic transfiguration then, to extrapolate on this principle of withering, is that, potentially, the necromantic adept could perpetually dwell in a state of thanatoic twilight in which one is voluntarily caught between the realms of the living and the dead as a thanatoic cohabitor. Borrowing from archaisms of middle English, the word "Lich" is used to describe a person that has achieved this state of living undeath. The original word was used to refer to a dead body -- a corpse -- and was the basis of such terms as a "lich way", a "lich gate", a "lich walk", a "lich wall", a "lich wake", and other things concerned with the dead. Given that the word refers not only to the corpse itself but also things related to the dead (many of those terms refer to structural parts of a proper cemetery) the modern abstract "lich form" is used to describe those that have achieved a corpse-like state of necromantic undeath. Assumably, a person in perpetual lich form, though faded on this level in the form of ill health, is also faded though present in the underworld and able to call on powers from either side at will. Rather than simply touching the power of death when it is needed and treating it like an inactive widget, the lich state allows a practitioner to live as a thing half dead at all times and transcend the physical world by becoming a quasi-inhabitant of both realms.

This perfect balance of life and death which is threatened by the attraction that both the realms of life and death exert on the soul. Whereas Life can be metaphorically likened to a wave along a graph, seen to progress in strong and weak cycles which peak and plummet then rise once more, Death can be seen as the median line itself which Life follows -- a perpetual constant which does not waver. Using the powers of decay and becoming an active part of the realm of death in a somewhat transdimensional manner by shifting one's soul in much the say way as the Greek practice of katabasis formed, which is the travel of the soul to the netherworld, the idea of exploiting physical lichdom arose. The soul does affect the body, but the soul can also work independently of the body as understood by those that have various out of body experiences and near-death experiences, and as well by the necromantic practitioner who often attempts to separate body and soul intentionally (at least partially; full separation is rumoured to be the formula for a coma).

What are the implications of perpetually cohabiting the realm of the living and the dead?

It has many implications and potentials that the necromancer sees as beneficial for practical purposes. The first of these is the notion that while the material body in the realm of the living may become frail, a frail shade of the self is also projected in the realm of the dead to always be available to interface with the netherworld. This is advantageous because the spells of necromancy would become less of an event and more of a way of life (forgive the pun). Living undeath, in this sense, mirrors the mythical tales of the vampire's condition of wading through life as the living dead and supports the stereotype attributed to necromancers throughout the ages as withered corpse-like beings who are obsessed with the nature of death (or, in the case of Socrates, for example, being obsessed with the nature of the soul). Spells can be called upon directly by using one's own soul rather than calling upon a random geist or the whims of the current dead due to the transcendent state. Gaining familiarity with the mechanics of undeath is supposed to be the key to performing one's own tasks through the spiritual realm with the highest degree of precision since there is no chance of instructions being misunderstood and resulting in effects which are not precisely what the necromancer commanded. The process of assigning tasks for the ensnared spirit can be circumvented entirely by simply performing these duties directly through the lich state. The ability to fissure spirit from flesh also carries other potentials such as preparedness for a spell at any given instance rather than having to "cast" a spell, per se. Being one of the living dead allows for the entire world to become a hazy life/death conglomerate in which both worlds and their interactions are seen at all times and can be instantaneously compared and contrasted to gain a deeper understanding of how the soul affects the world. It carries implications of the power of remote viewing, empathy, and perhaps even the ability to influence the unwary soul of those one might encounter in daily life without their knowledge. The ability commonly known in occult circles as "second sight", which is the ability to "see" magic as it occurs, is also supposed as a natural skill for those that have attained lich form. In this sense, lichform is a necromantic state which allows one to predict or precipitate future events by knowing the past through interactions with the realm of the dead and the denizens thereof. This list does little to even begin to tap the dreams that many necromancers hope to bring to fruition by achieving the coveted lich form state.

As has been said, and can not be reiterated enough, this is called a theory for a very good reason. It is not something one picks up in a day or achieves in a year. The quest for lich form is the holy grail of necromancy and none are reported to have ever fully achieved it. It is the ultimate goal of many, and often facets of it are claimed to have been touched in much the same way that Gnostics have claimed to have achieved gnosis but have not managed to find a way to fixate the self in such a state. Experimentation continues in the field of lichform transcarnation to fully unleash the awesome powers of the realm of the dead within the realm of the living.