Fendahl

Nicknamed Eustace by Professor Adam Colby, who discovered it, the skull was carbon dated by Thea Ransome and discovered to be too old to fit modern assumptions about mankind's evolution.

In reality, the skull was the host for the Fendahl, a creature of pure evil, which came to Earth after its home planet was placed in a time loop by the Time Lords.

The Fendahl was the physical embodiment of death itself. The skull took over Thea Ransome, turning her into the Fendahl Core, preparing to reassert dominance over Earth. However, the Doctor stopped it, depositing the Fendahl Skull inside a supernova.