Erasmus Darkening

Erasmus Darkening was an alien scientist who came to Earth in the 17th century.

Creating the portal
In the 17th century, Erasmus Darkening posed as a magician and alchemist and was employed by Lord Marchwood. He used advanced technology to open a portal between dimensions to get home. Innocent people were caught in Darkening's experiments and trapped between the dimensions. They were thought to be ghosts haunting Ashen Hill Manor. Marchwood attacked Darkening after the disappearance of his children, impaling him with his sword and damaging his device. The accelerator became powered by the people it took and channelled their life energy into Darkening, granting him immortality.

Immortality
Darkening became obsessed with his immortality, and began to call the Manor his dominion. Marchwood would confront him again, and trick him into stepping into a electromagnetic field, ending his existence whereupon by stabbing his sword into the metallic floor engraving of the Marchwood family seal, which channeled the energy into it and trapped Darkening inside to be destroyed. His last words were, "Eternity... I've lost eternity!!" With Darkening vanquished, Sarah Jane Smith disabled his device once and for all. (TV: The Eternity Trap)