From a less memorable one in Invisible Enemy, to another one of my favourite stories (a good run it seems overall here with the 4th Doctor and Leela), though problem for rereading is I know this tale so well, it can colour some of the earlier scenes.
Quite a Lovecraftian tale here, to basically end the run of horror stories that were quite common up until this point since the Planet of Evil, and has a nice amount of mystery to begin with, and a nice moment where one of the characters realises to his horror quite what is / has been going on.
K9 pretty much absent for this story, but Leela and the Doctor are in good form here, helping show the horror of the situation, while at the same time showing their ability to come up with solutions and get out of trouble.
I like the nature / idea of the Fendahl here as well, definitely a Lovecraftian type of being as such, though could have been nice to see a bit more success on their part, or perhaps been set somewhere other than Earth to see more of an impact, in the vein of Horror of Fang Rock, but that is because I sometimes like to see such creatures have a win occasionally.
The ending perhaps didn't quite live up to the build up to that point, but at the same time did sell somewhat the nature of the enemy - not really an intelligent, deliberately evil foe, but more an instinctive, animal almost type foe, where the instincts at times let it down.
Overall though, a really great read for me :)