Therefore, its known narrative elements are not a part of the Doctor Who universe. It may have been the basis for a similar story in another medium, however — and that story may indeed be a part of the DWU.
The Ultimate Evil was a story planned for Season 23. Along with Yellow Fever and How to Cure It, it is one of the three stories of the planned season not to be adapted as audio story in Big Finish Productions' Lost Stories series.
The story would have been set on the holiday planet Tranquela, where Dwarf Mordant had been using a space ray to make all the good people raging killers.
The TARDIS would have arrived on the planet. Peri would have been attacked and left for dead, while the Doctor would have temporarily fallen under the Dwarf Mordant' powers.