Thirteen physically separate organisms make up the Fendahl.
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.
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 affect the electrical conductivity and osmotic control of their cells, was fatal to them.
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 Humans 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.
Roughly 12 million years BCE, the Fendahl evolved on the Fifth Planet located between Mars and Jupiter. The Fendahl had begun 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. The Fendahl still survived and transported itself to Earth.
The Fendahl passed into legend, though, on Gallifrey. 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.
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.