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.
The Face of the Fendahl was an unproduced Reeltime Pictures film by David J. Howe. It would have been the third film to feature Kate Lethbridge-Stewart and Douglas Cavendish. The script was written, and was to be filmed in a castle in either Crete (as Howe told Downtime) or Cyprus (as he wrote in Talespinning). The plot was to be a Fendahl version of the film The Masque of the Red Death.
The return of Doctor Who to TV put a stop to the production being made, in addition to the separation of then-married stars Beverley Cressman and Miles Richardson. (REF: Downtime – The Lost Years of Doctor Who)
Barnaby and a Time sensitive companion named Catherine have been drawn to the First Crusade, where they're trapped in a decrepit castle by the cruel Prince Vincente. Catherine is left drugged and Barnaby in reluctant service. However, Barnaby is able to summon Kate Lethbridge-Stewart and Douglas Cavendish for aid: something that annoys both of them, though a disgruntled Cavendish thinks she agreed too readily.
The time travellers arrive at the castle, where Vincente greets them and claims his castle is all but deserted since the Crusaers arrived in Antioch, his staff recruited for the war; only a Turkish deserter named Ibrahim is there to serve. Kate and Cavendish are startled to realise where they are (he'll claim he's from the "Prussian army") and wonder how to get out of it. They split up, and Kate finds Barnaby, who reveals he's no longer a 'ghost' but without Catherine's help, none of them can time travel from this point.
Meanwhile, Ibrahim talks with Cavendish and reveals that the Crusaders' morale was boosted when they found a spear they believed was the once that pierced Jesus Christ - and he took it from the battlefield in the hope this would undermine them. Vincente appears to reveal he gave Ibrahim shelter in exchange for the Spear of Destiny. As the conversation plays out, a man is dying in the corner to Cavendish's concern; he watches Vincente carve a pentagram at knifepoint into the man while saying his master will arrive at the castle soon.
Cavendish and Kate compare notes, and she says Barnaby seems to be hiding something. She is not happy that he's drinking again to calm himself.
Vincente calls everyone (bar Catherine) into his hall for amusements. He encourages Ibrahim to try and sexually assault Kate, leading to a swordfight between Ibrahim and Cavendish - while Vincente cheers, Barnaby sneaks off to visit Catherine but she is unresponsive. The UNIT officer wins the fight but spares his foe. Vincente takes the pouch with the Spear off Ibrahim, which is revealed to actually be shard of bone, and using some of Kate's blood he carries out the summoning. Catherine rises from her bed, her eyes glowing gold, and is welcomed as Vincente's master: the Fendahl.
The Fendahl will claim whoever Vincente marks, and so pursues the cut Kate. Barnaby distracts it by using the bone shard to cut Ibrahim and so the Fendahl devours his life instead. Catherine regains her mind afterwards and says he was the eleventh victim in this castle: one more and things will be complete. Vincente cackles that this is excellent: he has three victims to choose from.
After Vincente leaves, Barnaby realises he's a member of the Sodality; he's time-travelled to the Crusades in order to use psychic energy to feed the Fendahl. The only way to stop is to trap it within folded time. A nearby ruined temple has lines of power that can be used to do this, and Kate and Cavendish are sent to make sure it's clear while Barnaby tries to retrieve Catherine. They're all unaware Vincente was listening in.
Kate and Cavendish have to evade Fendahleen to get to their destination but once there, Barnaby surprises them and holds a knife to Cavendish: he lied to them, in order to get Catherine back he needed to sacrifice one of them to the Fendahl. He pushes the man into the temple and waits for the nearby Vincente to emerge with the bone shard. He reveals he knows how the Fendahl works, claims he's a Sodality agent himself, and that he'll help Vincente sacrifice the other two in exchange for Catherine going free. Vincente agrees and uses the shard to summon the Fendahl within Cavendish.
Cavendish starts to transform into a Fendahleen creature and prepares to feed on Kate. However, Kate manages to draw Vincente into a struggle and cut himself with the shard: he has to flee for his life, the Fendahl in pursuit, through his castle. It corners him, mocking him that "I thought you wanted to meet your master", and drains his life. Meanwhile, the mentally free Catherine turns out to be a fiery figure who takes charge and Barnaby reveals he was lying to the enemy before, as he needed Catherine back to have the power to fix anything. She is able to channel the power of the place into a time loop. The Fendahl and Vincente are now trapped in the last few seconds on loop, an endless moment of death with no release.
Kate pleads for a way to rescue Cavendish. Catherine can do this if she has something like a fire to focus on - and Kate uses Cavendish's whisky flask for create one. The temporally retrieved Cavendish decides to give up on the drink, as long as Kate will help him. Both sets of time travellers depart for their respective timezones. 
Behind the scenes
- The script is available to read in Talespinning from Telos Publishing.
- Vincente was said in the script to be "very much based on" Vincent Prince in the 1960s horror films.
- The Fendahleen are humanoid figures in cloaks, with monstrous tentacled faces. This is different to how they're depicting in TV: Image of the Fendahl and the change is presumably to be more achievable on the Reeltime budget.
- The Spear of Destiny was allegedly discovered in 1098 and used to rally the Crusaders at Antioch, but was likely an intentional fraud in real life. 
- The Fendahl at work in 1098 Antioch and the bone-shared 'spear' were used in the Time Hunter novel Deus Le Volt. The Deus writer Jon de Burgh Miller is thanked at the end of the Face script.
- Talespinning collection by David J. Howe
- https://en.wikipedia.org/wiki/Holy_Lance#Antioch Wikipedia: Holy Lance, Antioch]