Erasmus Darkening

Erasmus Darkening was an alien scientist who came to Earth in the 17th century. He posed as a magician and alchemist and was employed by Lord Marchwood, and used advanced technology to open a portal between dimensions in an effort to get home. Innocent people who were caught in Darkening's experiments and trapped between the dimensions, were thought to be ghosts haunting Ashen Hill Manor. Marchwood confronted 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.

From this point onwards, 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, thereby ending his existence. Sarah Jane Smith then disabled his device once and for all. (SJA: The Eternity Trap)