Baron Rostov's opponent in this story is a lich, so let me define a lich for purposes of this story.
The beginning definition of a lich comes from Dungeons and Dragons. A D&D lich is defined as one of the most powerful members of the undead monster category. The lich has a corporeal body, is highly magical, and highly magic resistant as well. The body is usually tough and mummified, but capable of moving quickly and inflicting a lot of physical damage to an opponent. The lich is a master at spell casting and can also suck life energy the way a ghost can. They are often rulers of dark kingdoms, and leaders of small armies of horrible creatures. The Nazgul of Lord of the Rings are perhaps the most famous literary liches. Conan would take on liches from time to time as well. These creatures have a religious component as well, as they can be turned by clerics, but only the most powerful, and lucky.
I started with this, then extended it as follows...
Think of the 1990 movie Ghost. As the bad guy dies, black evil little spirits come up from below to take his soul away, and as the hero completes his burning quest, white spirits come from above and take him away.
Ghosts and liches are souls powerful enough to hold these afterworld "guide spirits" at bay. The spirits come, but the soul refuses to leave, and is strong enough to make that refusal stick. The guide spirits won't return without the soul they've been sent for. They will hang around, trying to weaken the soul's resolve and spiritual strength, and convince it to come with them, so they can complete their mission.
Time is on their side, and sooner or later the soul succumbs. However, if the soul is strong and wily enough, and has a strong enough sense of mission, that can be a long time. The difference between a ghost and a lich is the lich actually masters these guiding spirits -- while they are waiting, they do his bidding.
These guide spirits see and influence souls. Normally, their influence on souls in living people is minimal, but they can talk to souls in the living and thus influence living people's behavior. The lich can amplify their communicating power with living souls, and it sends these spirits as messengers to other souls. The result of spirit influence can be quick, simple and direct, as Obiewan and The Force did to the Storm trooper, or they can influence in more subtle, more enduring ways, such as changing a person's attitude through subconscious suggestions (the little devil on the shoulder).
It is not pleasant for a soul to be separated from a living body. The soul experiences "phantom limb". Phantom limb is a medical condition that real living human beings encounter when nerves to a limb are severed. There is no sensation coming from the limb, but the nervous system spontaneously generates sensations that feel as if they are coming from the limb. These can be a mild nuisance, or they can be serious, painful distractions -- such as the feeling that a muscle is cramped.
The soul is "wired" to function with a body the same way the human brain and nervous system is wired to function with limbs, so when the body is removed, lots of spurious sensations flood the soul's sensory input. This is enormously distracting. This is why most souls willingly go with guide spirits, and why most ghosts act so distant, distracted and crazy. All this distraction -- and it is often very painful distraction -- weakens the soul's resolve and it is the primary reason why even souls with a strong mission finally give up and go with the spirits.
Having a body to inhabit reduces the spurious sensations. This is why ghosts will try to move into someone else's body, and why the lich reanimates his or her own body. The body the lich inhabits is usually his or her own body, reanimated. In fact, the lich is usually the one who has his or her body mummified, so it won't rot away. A lich may inhabit someone else's body, but that's not nearly as satisfactory. It doesn't reduce the spurious sensations nearly as well.
The result in a lich is a creature who oscillates between periods of enormous determination, enormous control and enormous direction -- and periods when the distractions are overwhelming and produce a psychotic insanity: catatonia, hysteria, mania or depression.
Meshing high technology with magic produces the newest version of a lich: the brain-dead lich. In this special case, the lich looks and functions much like a living human. The body is warm, the skin and muscles soft and flexible, the heart and other organs are functioning normally. An experienced lich can pass a routine physical checkup, unless the doctor is looking for something unusual, such as no brain activity. What is dead is the brain. The brain is dead, and the soul should leave, but this is a lich, so the soul stays, and animates the body. In this special case, when the lich animates this body, he sustains it as a living body instead of a mummified body.
Sustaining the living body requires great intellect and attention as well as force of will. Some vague sensations from the body filter through, but these sensations are not nearly as direct and immediate as they are when the being is alive. It's like having leprosy. The lich doesn't feel hungry, sleepy, a full bladder or a full bowel, so these must be handled by "scheduling" rather than by waiting for an "interrupt." If the lich doesn't "pamper" his body, it will quickly show signs of wear and tear.
The lich "lives", but the important things in life are quite different from those of a human inhabiting the same body. Like the ghost, and unlike the vampire, the lich lingers on Earth to accomplish a single goal. Vampirish-like hedonism is extremely difficult for the lich, and he or she will engage in it only as a form of disguise. The lich is more likely to be a recluse and behind the scenes power-broker ala the Howard Hughes of the Howard Hughes legend.
The lich's helpers are elementals from the spiritual planes. In this story, they are imps. The lich turns them from their normal tasks and bends them to do his bidding instead.
These spiritual elementals come from many spiritual planes. Some of these planes are aligned with specific Earthly religions, and if a person has a strong sense of religion at death, a summons goes forth to that plane, and guiding spirits come quickly. A strongly religious soul is usually anticipating the trip, so the guiding spirits can lead it quickly to their spiritual plane. If a person is not strongly religious, the soul must wait for a wandering guiding spirit to discover it before it is lead away to another plane.
The average soul goes quickly bonkers after death -- suffering from confusion and phantom limb. When the guiding spirits discover the wandering soul of a long dead person, it's usually a "basket case" that doesn't do much -- just quivers helplessly and looks forward to any relief. The spirits easily control such a soul and lead it to one of the spiritual plains without fuss.
Occasionally, a soul keeps enough presence of mind after death to keep at a task. Such a soul becomes a ghost. Ghosts are souls the spirits cannot easily lead. Instead, like wolves waiting for a dying old moose, they will watch the soul and wait for it to become leadable. This will happen anytime the soul falls into deep despair and confusion, or immediately after the ghost's driving mission has been accomplished.
These spirits are not passive, they talk and tug constantly at a soul. The feeling is similar to old women at a border town farmers market -- talking and tugging constantly on prospective customers, and being quite obnoxious.
These guiding spirits talk to souls, but those souls still in a living body are pretty incomprehensible to the average spiritual elemental, so they don't interact much. A lich will ensnare a guiding spirit into it's service by offering it the gift of talking meaningfully to souls in living bodies. It's a "deal with the devil" for the spirit: the spirit learns something that for it is a great power -- talking to souls in living bodies -- but in return it must do the lich's bidding. The lich-guiding spirit relation becomes the same as Sauron and the Nazgul in Lord of the Rings. The guiding spirit becomes tremendously powerful compared to it's fellow creatures, but it must use those powers in the service of the lich.
The power is the power to communicate persuasively with souls in living creatures. While in the service of the lich, these elementals can be heard by living souls. There is a great range in communicating skills between spirits. Some can merely whisper to the soul, others can shout and badger or terrify the soul. Still others can insinuate ideas without the soul being aware the idea was externally generated.
The lich's existence is always tenuous. The being is constantly distracted, and if the iron force of its will ever lapses, the guiding spirits will finish their mission, and lead the lich's soul off. Young liches are quite vulnerable to distraction: they can be distracted or talked out of their existence. A lich who despairs will shortly be taken away. Old liches have strongly developed survival thinking, they are unlikely to "lose it" to simple physical or verbal distractions.
If the body of an old lich is destroyed, the soul will survive as a ghost if it's survival thinking is well developed, but it's power and ability to concentrate on external events will be considerably reduced. An ex-lich ghost, however, is much more virulent than the average soul-with-a-mission ghost. If it survives the shock of losing it's body, and doesn't despair, it will not stay a ghost for long, it will move into another body and continue it's mission.
Inhabiting a new body is not a quick process. It's like moving into a new home or setting up a new personal computer. It takes time to get things arranged properly, so the hero who destroys a lich's body rarely has to face the lich again.
The three most common ways to permanently dispatch a lich are to convince it its mission is complete, to throw it into serious despair, or to hit it with a blast of such power (often clerical) that it is momentarily stunned, confused, and without the will to resist the guiding spirits -- who will then cart it away.
Vampires and liches are both highly intelligent and potentially long-lived; both can be killed. In vampires the threat of premature death creates risk-adverse behavior. Vampires inhabit earth to enjoy existence. Mature vampires habitually back down from any confrontation that is less than overwhelming in their favor, and strongly prefer having subordinates make a situation overwhelming before they get involved.
"There are no old, bold vampires." mature vampires constantly intone to their initiates. Those that learn this lesson well become mature vampires themselves.
For liches this truism is irrelevant. They are driven beings. They exist to fulfill their mission. When they are "off duty" -- unable to take further action to fulfill their mission -- they can act as risk adversely as vampires, but the first question asked of any action, or inaction, is: Will this further my mission?
Liches and vampires will both seem shy and retiring while they size up novel situations. Vampires will stay that way until they see great advantage or low risk in doing otherwise. Liches, on the other hand, may take quick and risky action when something about the situation "clicks" concerning their mission, and they see taking quick, risky action as advancing their goal.
These are the ideas that I used to construct the personality of the lich, Johanne Porter, in this story.