The Fendahl was an entity that devoured life itself. It originated on the fifth planet of Earth's solar system, which the ancient Time Lords placed in a time loop in an attempt to imprison the creature. However, the Fendahl escaped and, in the form of a humanoid skull, was buried under volcanic rock on prehistoric Earth 12 million years ago. The story of the Fendahl passed into Time Lord myth, and was forgotten. The Fendahl's power, contained in a pentagram-shaped neural relay in the bones of the skull, affected life on Earth via a biotransmutation field, influencing life forms in its vicinity (including the early hominids) to develop into forms it could use.

In the late 20th century, the Fendahl skull was discovered in Kenya by a team of anthropologists under the leadership of one Dr. Fendelman. Fendelman brought the skull to an English research facility at Fetch Priory, near the village of Fetchborough. The Priory was built on a time fissure, causing psychic ability in some nearby residents. In the Priory, Fendelman and his fellow researchers Thea Ransome, Adam Colby and Maximillian Stael performed experiments on the skull, attempting to unlock its secrets. Fendelman used a crude time scanner to examine the skull, a dangerous activity which drew the attention of the Fourth Doctor. Stael attempted to capture the power of the Fendahl for himself by means of black magic rituals, performed with the aid of a local coven, but he, Fendelman and Ransome were all being used by the Fendahl to recreate itself.

The Fendahl was a gestalt creature with multiple aspects. Thea Ransome was transformed into the Fendahl Core, a humanoid female with golden skin and blank, staring eyes. Several of the cult members became slug-like creatures called Fendahleen. All the aspects of the Fendahl had powerful psychotelekinetic ability, and can control the muscles of human victims. The Fendahleen were vulnerable to sodium chloride, which altered the creatures' conductivity and destroyed their electrical balance.

In its final form, the Fendahl would consist of the Core and twelve Fendahleen; however, the Doctor was able to prevent the creature from reaching its full manifestation. He rigged Fendelman's time scanner to implode, destroying the Core and the Fendahleen. He also removed the skull, planning to drop it into a star about to go supernova.

The Fendahl has also appeared in the Eighth Doctor Adventures novel The Taking of Planet 5 by Simon Bucher-Jones and Mark Clapham, where a group of Time Lords from the Eighth Doctor's future attempted to release it from the time loop trapping Planet Five, only to learn that an even deadlier life-form had evolved inside the loop; the Maemovore, a devourer of concept itself, which the Doctor was only just able to defeat by convincing a group of TARDISes from the future to help him expel the entity from the universe. The Fendahl also returned in the Kaldor City series of audio plays and the Time Hunter novella Deus Le Volt by Jon de Burgh Miller.