Maximillian Stael was the leader of a coven and a scientist who, along with Adam Colby and Thea Ransome, worked for Dr Fendelman in studying "Eustace" at Fetch Priory. He concealed his membership of the coven from his colleagues, including Fendelman, whom he was assisting in secret experiments, working with a time scanner.
In July 1977, a hiker was found dead in the woods adjacent to the priory. Fendelman asked Stael to "get on to London" and tell Hartman that he wanted a security team present within two hours. He also asked Stael to perform a post-mortem on the hiker's body. Stael discovered that the body was rapidly decomposing, as if the energy had been removed. He told Fendelman "all the binding force has gone and all that remains is a husk".
Ted Moss, a member of Stael's coven, visited him to warn him of the arrival of the Fourth Doctor and Leela, telling him that they "know all about us". Stael said he would deal with them and asked if their friends were prepared. Moss said they were waiting for the word. Stael told him that, when the time came, they must be twelve, but Moss disagreed, telling him "we know you lead the coven now, but we know the old ways. Thirteen be the number". When he had gone, Stael spoke to himself, saying "a place must be left for the one who kills."
When Stael found Thea in Fendelman's lab, he told her it was fitting she should be the key to his power - a link between her and the Fendahl having been created - and, declaring her the "chosen one", he drugged her with a cloth pressed over her face.
When she woke, Thea was in the cellars of the priory. Stael told her she would be "the medium through which the ancient power of this place is focused" and that, through her, he would "conjure and control the supreme power of the ancients". He then injected Thea so that she would sleep. Before the drug took effect, she called him a fool, to which he responded: "I shall be a god."
Returning to the lab, Stael used a gun to threaten Fendelman and Colby, ordering them to turn off the time scanner as he was not ready, which the latter did. The pair were bound to pillars in the cellars of the priory, next to where Thea was laid out on a pentagram on the floor. Fendelman asked Stael how long he had been planning this, and he replied that it was ever since Martha Tyler's vision began to come true. Colby mocked him, calling him "an occult freak. One of those feeble inadequates who thinks he communes with the devil." Stael replied "unlike you, I am not a crude lout, Colby. The grimoires do not impress me. Mrs Tyler's paranormal gifts and the race memory she draws on, these were the signposts on the road to power." He told Colby he looked forward to his terror.
Stael placed the skull, Eustace, on an altar and connected it to the time scanner. The members of the coven gathered around the prone Thea. Realising his intent, Fendelman warned him not to carry out his plan, telling him that "mankind has been used." Stael shot him dead. The ritual began and Thea was transformed into a Fendahl core. She set about killing members of the coven as Stael protested "this is not how it should be".
The Doctor and Leela went into the cellars and freed Colby. The Time Lord tried to help Stael to escape but, having looked into the eyes of the transformed Thea, he could not move. Stael asked the Doctor to hand him his gun from altar. The Doctor told him it would have no effect on the core, and he replied that it was for him, not her. Having given him the weapon, the Doctor left the cellars, and Stael shot himself. (TV: Image of the Fendahl)