Pentagram

A pentagram was the symbol of a five-pointed star. It was often associated with evil in certain Earth religions.

The skull of a Fendahl Core bore a pentagram, seen by x-ray, that was a neural circuit within which it held an enormous energy reserve. Because of this, as long humans could remember, the pentagram has been a symbol for mystical energy and power. Thea Ransome was later transformed into the new Core by being drugged and placed on a pentagram, from which she arose dressed in beige robes with golden skin. (TV: Image of the Fendahl)

The Scourge could possess people, existing within them as a burn mark on their hands in the shape of a pentagram. They referred to this as "the shadow" and resented people touching it. (AUDIO: The Shadow of the Scourge)

The pentagram eventually melded with the "A" of Anarchy as part of counter-culture. (PROSE: Love and War)

Information from invalid sources
A pentagram appeared on a laptop featured in a case file recorded by Giles. (NOTVALID: PROBE: CASE FILES - Vol 2)