Fendahl

The Fendahl were a gestalt organism, 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.

Biology
Thirteen physically separate organisms made up the Fendahl.

Fendahl Core
Fendahl Core, 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, Thea Ransome appeared as a golden-skinned, majestic being with gigantic, unblinking eyes.

Fendahleen
Each Fendahl Core had twelve Fendahleen, large eyeless and limbless creatures with a fringe of pink tentacles around and two large fins above their mouths.

Table salt, which affected the electrical conductivity and osmotic control of their cells, was fatal to them. (DW: Image of the Fendahl)


 * It is unclear if the Fendahl Core has the same vulnerability.

Life cycle
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 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 was killed. All of the parts of the Fendahl can telekinetically control people, but the Core has many more psychic powers such as mind control and astral projection.

The Fendahl and the Time Lords of Gallifrey
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. However, the Fendahl survived and transported itself to Earth.

On Gallifrey, the Fendahl passed into legend. As a boy, the Doctor heard stories about the Fendahl that caused him to fear it even as an adult.

The Fendahl's natural enemy was the Fendahl Predator, which also originated on Planet 5. (EDA: The Taking of Planet 5)

The Fendahl and Humanity

 * We do not know if the Fendahl physically transported itself or projected itself in the form of energy. The Doctor thought it must have ravaged Mars along the way, although twelve million years later, Mars did still possess life in the form of the Ice Warriors.

The essence of the Fendahl lay dormant in the skull of the Fendahl Core. This skull was able to affect the development of Humans in order to produce a species which it could use. The skull was eventually discovered by Dr. Fendleman in the 1960s by discovering a "sonic shadow" and tracing it back to its source. Fendleman found the skull fossilized in the lava of an inactive volcano in Kenya. Since, according to Human archaeological theory, a twelve million year-old Human skull should not exist, Fendleman deduced, correctly, that the skull must have come from space. (DW: Image of the Fendahl)