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.
Horror of Det-Sen was the planned second novel in the Lethbridge-Stewart novel series. Penned by Lance Parkin, the book would have featured Alistair Gordon Lethbridge-Stewart and Edward Travers returning to Tibet. It was cancelled due to scheduling issues, and is currently not in planned production.  The first several pages were printed in the back of The Forgotten Son.
Earth is under attack.
London is recovering from the Yeti invasion. Recent events have starkly demonstrated to Colonel Lethbridge-Stewart that the force behind the Yeti, the Great Intelligence, remains an active threat. To have any hope of defeating it again, he needs to know his enemy.
Forty years ago, the Great Intelligence occupied the Buddhist monastery of Det-Sen, high in the Himalayas. Now Lethbridge-Stewart plans to go to Det-Sen, in an attempt to scour the site for clues to the enemy's nature and capabilities.
Rather to his surprise, Professor Travers, who was there as a young man, insists on coming with him.
They are not the only people interested in the Det-Sen monastery. A diabolical genius is keen to tap into the power of an ancient, dark god, and he has sent his henchmen across the world to collect the artefacts the Great Intelligence has left behind.
What is The Horror? Will it mean the end of the world – or are things much worse than that? And … what's that sound? Can't you hear it? That electronic sound? No, it's still there. You can't hear that? Really?