Dion Fortune says that she told the story titled "Blood-Lust" (in The Secrets of Dr Taverner) exactly as it was played out in physical and psychic reality.
Captain Craigie, back from war, starts killing for blood. He starts by raiding chicken coops and works his way up to sheep. Along the way, he can't help trying to sink his teeth into his fiancee's neck. He tells Dr Taverner, the unusual practitioner who turns his attention to the case, that he suffered "shell-shock" in the trenches. This was a familiar term in the World War I era; today we are more likely to speak of PTSD (post-traumatic stress disorder). But in this case, "shell-shock" seems an exact description. Captain Craigie was blown out of a trench.
The source of the problem becomes literally visible when people start seeing the apparition of a German soldier in a flat cap and field-gray uniform. This, we learn, is the etheric or dense energy body of a dead enemy soldier who has attached himself to the unfortunate Craigie, seeking to feed his "vitality hunger" (as Taverner diagnoses his condition) through his living host.
Dr Taverner's solution involves baiting a trap, by nicking the neck of the captain's girlfriend. While Craigie is physically restrained, the hungry ghost of the German soldier leaves his body to seek the fresh blood. Taverner stands between the ghost and its host and forces it into "a psychic killing-pen" in the form of a triangle.
Dion Fortune is discreet about what happens next. Taverner makes "a Sign and a Sound". The gray etheric form spins into a whirling spiral and disintegrates. Captain Craigie is released from the hungry ghost that was riding him,
Dr Taverner pronounces that the source of the problem was "a corpse who was insufficiently dead."
While some parts of the plot summary may sound like the script for one of those old Hammer horror flicks, the story touches on themes of perennial importance, including the plight of veterans of foreign wars yesterday, today and tomorrow, who may be carrying unwanted energies of the dead.
One of Dr Taverner's lessons, in this story, is that we have more than one physical body. To be specific:
We have two physical bodies...the dense material one...and the subtle etheric one, which inhabits it, and acts as the medium of the life forces, whose functioning would explain a great deal if science would only condescend to investigate it.
By my observation, this is exactly correct. Failure to understand the nature of the etheric body and its survival of physical death confuses our relations with the dead and delays or prevents recognition of the need for spiritual cleansing and releasing in healing cases of PTSD, addiction and other disorders.
Dr Taverner instructs that "it is possible to keep the etheric body together almost indefinitely if a supply of vitality is available". In "Blood-Lust", the supply of vital energy comes through "a human feeding bottle" that feeds on others in an effort to replenish itself.
How common is the problem? Well, literal vampirism and blood-drinking may be rare (despite the contemporary vampire fad) but energy vampirism has never been uncommon, alas. Dr Taverner, the magician, alludes to dark side magical practices by those who seek to avoid the second death (of the etheric body) by forming "a connection with the subconscious mind of some other soul that still has a body." He cautions that "the lower type of medium" is especially vulnerable to this type of parasitism. But "higher" types can be vulnerable if in a weakened or absent condition, as when Captain Craigie was blown out of his trench, and (for a period) out of his body.
Who ya gonna call, if you don't have a Dr Taverner down the street? Well, I give rather specific guidance on spiritual release, including staging a "second burial" for an etheric body that does not belong with the living, in The Dreamer's Book of the Dead. And we have Dion Fortune's classic Psychic Self-Defence.