Well the question was answered without me asking it.
The question was does a twist ending like the suspected big bad turning out not to be really evil have warrant within a published work calling itself Lovecraftian?
OK, i know you said you had this, but I had already come up with a suggestion, so maybe you'll find some other use for it: if they are as efficient as you say about tearing down older construction and putting up new within weeks, it would stand to reason that they are generating a lot of construction waste, which has to go somewhere. Maybe there is a landfill somewhere with all this stuff that baddies set up shop in, proping up chunks of wall and scrounging old piping, or digging out modest tunnels throughout as they excavate for useful cast-off.
Well Korean law has it set up where most everything is recycled in some way. there are a few places where there are mounds of garbage and waste, but they are realtivly small in comparison to the size of the city. I probably will use them in some context but not as the main hiding place. Possibly foraging grounds for food or followers.
That sounds awesome but Im not seeing your question. I have a few though: How did such a powerful being get forced into said role? Ancient Mystical Binding spell? Did he make a bargain and get the shaft? Is there some even more Uber entity out there antagonizing him? Did humanities growth/presence inadvertently screw something up and somehow trap him? If he is shouting for help, how will such relatively tiny and fragile creatures be able to help him?
I a little torn on how it got stuck where it is. I have two ideas. The most logical would be that it was already imprisoned here centuries ago and humanity discovered it and its jailors (the lesser abominations). They were driven mad by the revelation of finding this place and mistook it for a fallen god and the jailors saw no reason to dissuade them and in fact played along in order to spread corruption. What actually imprisoned it would be left to mystery which should inspire fear given how strong this thing is and there is something out there that bitch slapped it into a jail cell. The other is that the creature was imprisoned by the mortals who now worship it, though most of them are unaware of this fact. They managed this because the creature has something that already partially binds it to the world, something it has to protect. The cultists used this to force it into a binding.
In either case the creature does have something that it is protecting that is key to the story.
As for back story, o_O, the creature is very powerful but it also has a job and can only act within the confines of performing that task. It is otherworldly but is cut of from that other place from which it spawned. Thus its choices in asking for help are very very limited to what is around it. Humans or animals. It is also not the only one of its kind though this will not really come into play much in the story. Most of these others are not imprisoned but many are. I think the city infrastructure could be used as part of their bindings and dive into the conspiratorial cults and secret societies that have influenced the construction of several major cites in the world.