Over twelve million years ago, on the fifth planet of our solar system, between Mars and Jupiter, evolution went up a blind alley. Natural selection turned back on itself, and a creature evolved which prospered by absorbing the energy wavelengths of life itself. It absorbed the full spectrum of energy - what people call the lifeforce, or soul. The creature - called the Fendahl - consumed all life on the Fifth Planet, even the life of its own kind. The Time Lords destroyed the planet and put it into a time loop, making it and all records of it invisible - but they were too late. Gallifreyan mythology maintained that the Fendahl had perished when its planet broke up, but in fact, the Fendahl had escaped by using its stockpile of energy to project itself across space in a manner akin to astral projection. It went to Mars first, and presumably devoured all life there. Then it continued to Earth, which was still several million years away from developing life, so the Fendahl went into a kind of hibernation.
All the energy amassed by the Fendahl was stored in a skull - human in appearance but, in fact, alien. The exact origin of the skull is unknown - it may be the actual skull of the Fendahl itself, or it may be a skull from one of the Fendahl's hosts. At the back of the skull, the bone structure formed a pentagram along natural fracture lines, and presumably the Fendahl stored its energy in this symbol. This energy was then dissipated slowly as a biological transmutation field, and any appropriate lifeform that came within the field was altered so that it ultimately evolved into something suitable for the Fendahl to use, thus possibly explaining the dark side of man's nature.
The form that the Fendahl apparently needs to survive, if the skull in which its energy is stored gives any indication, is a humanoid form. The Fendahl directed the evolution of mankind so that its ultimate form was the precise humanoid form it needed, and then fed into the RNA of certain individuals the instincts and compulsions necessary to recreate. These instincts were fed through the generations until they culminated in the precise individuals needed to bring about the Fendahl's rebirth.
Over time, the Fendahl skull became buried on Earth, until it was found in twelve-million-year-old volcanic sediment in Kenya by a scientist named Adam Colby, who excavated and reconstructed it. Colby was associated with a scientist named Dr Fendleman - "man of the Fendahl", although he was unaware of it - who had noticed a vibration, a kind of sonic shadow, while working on a missile guidance system. Fendleman became interested in the origins of man, and believed that by using a sonic time scan, he could establish that mankind was actually the product of an alien traveller who died on Earth and left its skull behind. He used the time scanner to see into the past, using it only after dark so as to minimize solar disruption, and believed that he could trace back to the moment of his alien traveller's death. At that moment, there was an enormous surge of power, which Fendleman believed to be an inpouring of energy, a concentration of power which was stored in the skull. He believed the pentagram on the skull to be a neural relay, and that the energy stored within it could only be released through the intelligent application of applied advanced technology. If this was accomplished, a signal would be sent to the traveller's race, initiating first contact between its people and mankind. This was, of course, not at all true.
His experiments were located in a priory in the woods outside the town of Fetchborough, the site of a time fissure which caused rumors of hauntings and apparitions, as well as telepathy and precognition in the people who grew up near it. One such person was an old woman known as Grandma Tyler, who had visions of the Fendahl as a huge dark creature which called her name and was hungry for her soul. In a dream, she saw it in a female form.
The reason for this was that the skull caused an inductive biological transmutation in one particular individual. Exactly how it picks this individual is unclear, but it seems to establish a mental link with its prospective host. The skull also becomes a mutation generator, using appropriate genetic material to recreate itself, and it seems to get much of this material from its host. When the Fendahl began its rebirth in Fetchborough, it chose scientist Thea Ransome, who began entering into trances when she was in close proximity to the skull. At one point, while unconscious, her body was surrounded by a golden aura and two embryo Fendahleen briefly appeared on her body. Gradually, the Fendahl took more and more control of her.
During the course of Fendleman's experiments, the skull began to absorb the energy released when the time scanner damaged the local time fissure. Not only did this precipitate its conversion of Thea, but it also enabled the Fendahl to begin consuming life again. It is so devastatingly powerful that within a year of its rebirth, it would reduce the population of Earth from four thousand million people down to one - its Core.
The Fendahl is a gestalt creature, composed of a Core and twelve Fendahleen. The Fendahl turns its host into the Core - in this case, Thea Ransome became a golden-skinned woman with large eyes, dressed in flowing golden robes, with long golden fingernails and an ornate ringed hairstyle. The Core is practically invulnerable, not harmed by guns at all. She has but to make eye contact with an individual, and they are put immediately under her thrall, practically helpless to resist her. Once she has gained control of an individual, a wave of her hand turns them into one of the twelve Fendahleen needed to compose the whole Fendahl creature.
The Fendahleen are tall green wormlike creatures with flaps running the length of their segmented bodies, and a flat, broad tail that leaves behind a trail of green slime. At the other end of their bodies, in place of a head, is only a large gaping mouth full of waving red tentacles. The Fendahleen make a sucking noise, along with a more prominent gobbling sound, as though they are constantly trying to extract and consume life. The Fendahleen are psychotelekinetic, able to control the muscles telekinetically. Primarily they paralyze their victims' legs so that they cannot move while the Fendahleen glide up to devour their life. People killed by the Fendahleen are left with a blister at the base of the skull (perhaps they draws the lifeforce out through this point of the human body), and with all the binding force gone, leaving only a husk, the remaining body begins to decompose rapidly. Unlike the Core, the Fendahleen do have their weakness - rock salt, particularly when shot up their throat, affects the conductivity, ruins the overall electrical balance and prevents control of the localized disruption of the osmotic pressures. In short, salt kills them, causing wisps of smoke to come out of their mouth. Surface contact with salt does not seem to have the same effect, but they do not seem to find it pleasant. When injured, Fendahleen squeal.
When the skull is activated, it begins to pulse with a golden glow from inside, and touching it causes great pain - apparently mental pain more than physical - although the cause of this is unclear. It can, however, be safely handled with gloves. Once the skull finds a host which can be transmuted into the Core, it begins to restructure the host's brain, and at this point it can create embryo Fendahleen out of pure energy. It seems to need to consume at least one person in order to create a Fendahleen, but its behavior is the same as a Fendahleen even in this noncorporeal state - it is accompanied by a hungry-sounding noise and paralyzes its victims. (Presumably the Fendahleen also create a blister on the neck when they kill, although the blister is only mentioned in conjuction with the state of the Fendahl before it has created any Fendahleen.) It even has some psychokinetic power which it can use to open doorways as well as paralyze muscles.
The Fendahl engineers its rebirth through several people - Colby, who excavates the skull; Fendleman, who activates it with the time scanner; Thea, who becomes the Core; and Stael, a deranged scientist who believes that he will be able to summon and control the ancient power in the forest, becoming a god. He leads a local coven who supply the Core with the remaining lives it needs to create all twelve Fendahleen. Once this stage of the rebirth begins, the skull starts to glow a bright white. The Core seems to be able to create Fendahleen without killing anyone, but perhaps it is able to store extra energy from previous killings for such a purpose (although the Doctor seems to believe that there is a one-death-to-one-Fendahleen ratio only). It is possible that the Core transforms some of her own biomass into a Fendahleen, as the Fendahleen seems to temporarily replace the Core, and the Core becomes somewhat intangible afterwards. The Core can teleport herself and at least one Fendahleen in at least a limited radius around the skull. But as long as one does not look in her eyes, she can be passed harmlessly through (although if the biomass-transformation theory is correct, that may be the only time when passing through her is safe).
The Doctor is able to rig Fendleman's device to set off a controlled implosion. He then isolates the skull inside a lead-lined box and removes it from the Priory before the house, along with the Core and her Fendahleen, are destroyed in the implosion. With that done, the Doctor has only to drop the skull into the intense heat of a supernova, which he believes will finally obliterate it. He finds an appropriate supernova in the constellation of Canthares, and presumably the menace of the Fendahl is destroyed forever.
