When The Enemy Within campaign was first introduced by GW, the penultimate episode was not planned to be Something Rotten in Kislev. It was to be The Horned Rat. There was, as far as I recall, no further mention of the adventure as part of The Enemy Within in any GW publications, except for an allusion in Warhammer City:
I see darkness gathering around a walled town – Shadows Over Bögenhafen! I see the Lord of Death astride a great river – Death, on the Reik! I see a hooded evil behind the seat of a once mighty lord – a Power Behind the Throne!
I see The Empire in Flames! The Horned Rat doth sit on the Imperial throne! It is all written in the Book of Changes. Yea, Chaos’ most determined enemies shall prove its greatest servants.
The Enemy is Within!
– Warhammer City, p83
During the Flame era there were advertisements for an adventure with the same title, but this was a different scenario altogether, and not part of The Enemy Within.
The Horned Rat – A huge adventure in which you get to play the bad guys as a band of Chaos Champions battles through a vast lair of the rat-headed skaven in the midst of the Chaos wastes.
– White Dwarf 115
The original scenario remained a mystery until Phil Gallagher shed some tantalising light on it:
The Horned Rat idea was all mine. Mine, I tell you. Who knows how it might have panned out, but I had this idea that a bunch of Skaven were developing a means of bringing Morrslieb down to earth… and/or they had created a portal to enable them to teleport to the surface to mine it… Chaos-mutations a-plenty!
Phil Gallagher, Graeme Davis’ blog
It would be interesting to know how the plot of the adventure might have played out, or if any work was done on it, but that information might now be lost in the mists of time.