The Fendahl were gestalt organisms from the original fifth planet of the Sol system. They were composed of two parts, a Fendahl Core and the Fendahleen. The Fendahl posed a threat to all life, and when one Fendahl Core and twelve Fendahleen coexisted they had the power to destroy all life on a planet.
Thirteen physically separate organisms made up the Fendahl, twelve Fendahleen and the Core. Additionally, a High Priestess of the Fendahleen would merge with them to complete the Fendahl creature. (PROSE: Doctor Who and the Image of the Fendahl)
The skull of the Fendahl Core which transported itself to Earth resembled a human skull in every respect, but for a pentagram shape in the forehead, visible only by an x-ray. After her transformation into the Fendahl Core in July 1977, Thea Ransome appeared as a golden-skinned, majestic being with gigantic, unblinking eyes, long golden fingernails and an ornate ringed hairstyle. The Core was practically invulnerable, not harmed by guns at all. When the skull was activated, it began to pulse with a golden glow from inside, and touching it caused great pain — apparently mental pain more than physical — although the cause of this was unclear. It could, however, be safely handled with gloves. (TV: Image of the Fendahl)
Each Fendahl Core had twelve Fendahleen — large eyeless and limbless creatures having a fringe of pink tentacles around their mouths, and two large fins above. Their weakness was salt, particularly when shot up their throat. It affected the conductivity, ruined the overall electrical balance and prevented control of the localised disruption of the osmotic pressures. In short, salt killed them, causing wisps of smoke to come out of their mouths. Surface contact with salt did not seem to have the same effect, but they did not seem to find it pleasant. When injured, Fendahleen squealed. (TV: Image of the Fendahl)
Fendahls were psychevores, meaning they fed on psychic energy. (PROSE: The Left-Handed Hummingbird) The Fendahl could drain all the spectrum of energy from their prey, effectively sucking out their souls. Even the skull nicknamed Eustace, the fossil remnants of a Fendahl Core twelve million years dead, had the power to drain the life force of the Fourth Doctor. The Core could create these Fendahleens from living beings with the correct genetic inheritance and in some cases could create the Fendahleen out of pure energy.If the Core was destroyed, the Fendahl were killed. All of the parts of the Fendahl could telekinetically control people, but the Core had many more psychic powers such as mind control and astral projection. (TV: Image of the Fendahl)
Roughly 12 million years BC, the Fendahl evolved on the fifth planet, located between Mars and Jupiter. The Fendahl began to kill all life on the planet, including each other. When the Time Lords learnt of the danger that the Fendahl posed, they placed the planet in a time loop. They also removed all trace of the Fendahl from their records, leading it to pass into legend.
However, a Fendahl survived and was able to escape the planet in the form of a skull. (TV: Image of the Fendahl) In its travels it caused damage to Mars, leading to mass extinction. (TV: Image of the Fendahl, PROSE: GodEngine) It eventually came to Earth, arriving in Kenya. This skull was able to affect the development of humans in order to produce a species which it could use. (TV: Image of the Fendahl)
The skull was eventually discovered by Dr. Fendelman in the 1960s by discovering a "sonic shadow" and tracing it back to its source. He brought it back to Fetch Priory, which had a time scanner. The Fendahl was able to grow in power using the time scanner, killing several people and possessing Thea Ransome. A local coven, led by Maximillian Stael, believed they could control the Fendahl and tried to help it, but were instead turned into Fendahleen. The Fourth Doctor was able to stop the Fendahl by overloading the time scanner and removing the skull, planning to dump it in a supernova. (TV: Image of the Fendahl)
During the War with the Enemy, Time Lords from the Eighth Doctor's subjective future had planned to use a fleet of War TARDISes to undo the time barriers around Planet 5 in order to release the Fendahl, but it was discovered that the Fendahl had been killed when time inside the loop was accelerated, resulting in the last other life-form on Planet 5 evolving into the Memeovore, a devourer of concept that had evolved to kill the Fendahl itself. (PROSE: The Taking of Planet 5)
The Fendahl also was successful in duping Reynald, former Earl of Marseille, into summoning it in Jerusalem in 1098, by using a bone spear imbued with its life-draining properties to draw nourishment, allowing it to transform him into a Fendahl Core and create several Fendahleen. However, due to the efforts of Simon, a knight of the period, Reynald was aged well into a decrepit state, weakening his ability to command the Fendahl's powers, killing both and banishing the Fendahl. (PROSE: Deus Le Volt)