"Eustace" was the nickname for an apparently human skull found in Kenya by Dr. Fendelman, who had discovered a "sonic shadow" in the area. It was found in the remnants of a volcano. Fendelman discovered two anomalies connected with the skull. It was twelve million years old, much older than archaeologists had estimated humans had existed. X-rays revealed a pentagram-shaped pattern in the crown of the skull. Fendelman experimented with the skull, determined to unravel its secrets.
Eustace had originated as the skull of a Fendahl Core and still possessed vestigial life. Glowing with power, it drained life from the Fourth Doctor. Fendelman's employee and the leader of a coven, Maximillian Stael, used it as the centrepiece of a ritual successfully attempting to imbue Thea Ransome with the power of the Fendahl, turning her into another Fendahl Core. The Doctor put it into a lead-lined box and took it into the TARDIS, so he could throw it into the heart of a supernova, in the constellation of Canthares to destroy it. (TV: Image of the Fendahl)
This not only failed to destroy Eustace, it strengthened it. After much drifting through space, it fell onto an unnamed planet and was taken to Kaldor City, where it was presented by a cult as the skull of Taren Capel, resulting in a new manifestation of the Fendahl. (AUDIO: Checkmate)