An altar was a structure on which offerings were placed. Maximillian Stael's coven set up an altar in the cellars of Fetch Priory. The skull, Eustace, was placed upon it and coupled to Dr Fendelman's time scanner. Thea Ransome was transformed into the core of the Fendahl and set about killing members of the coven. Stael was unable to flee, having looked into Thea's changed eyes. He asked the Fourth Doctor to hand him his gun from the altar, which he did, and then committed suicide.
By using a scanner beam and salt to confuse and repel the Fendahl, the Doctor was able to take Eustace from the altar and place it in a lead-lined box. As Fetch Priory was engulfed in the flames of the Doctor's controlled explosion, the core appeared above the altar, fading and reappearing slowly. (TV: Image of the Fendahl)