Therefore, its known narrative elements are not a part of the Doctor Who universe as we, on this Wiki, choose to define it. It may have been the basis for a similar story in another medium, however — and that story may indeed be valid.
Hex was a Doctor Who TV serial written by Peter Ling and Hazel Adair for Season 21. It would have featured the Fifth Doctor and Peri Brown. It was eventually adapted by Paul Finch into the audio story Hexagora.