It is funny that you paint yourself as lacking imagination but have built an interesting picture so far. I agree with other posters that there are some details that need some working, but I do like the whole idea.
Do you like/have intention to use any of the published adventure paths? If so, I think you would not have much trouble adapting some of them to your campaign. Specially if all you need is ideas, and you can pull yourself to do the necessary adaptations to match the original adventure path to your own campaign. A big threat from "beyond" is already the theme of Princes of the Apocalypse (where these barbarians could be dealing with one of the elemental cults), Tyranny of Dragons (they could be facing cultists sacking around) and Out of the Abyss (excursions from the underdark). The good about using the published material is that it is easier (at least for me) to adapt something than it is to create. And also, there are already a lot of maps, dungeons and encounters ready to be used.
If you want some inspiration from outside D&D, the 13th Warrior (movie)/ Eaters of the Dead (book)/ Beowulf might be a good source. The movie is based on the book, which in turn is partially inspired by the legend. There you could have a cult or some different culture living underground/inside the mountain, and they don't even need to be some sort of "evil", but instead a very different culture with their own alien customs (some of which might be very questionable, nonetheless).
Edit.: It is worth pointing to the obvious, that is: Dragons! It is in the name of the game, after all. You could have an old scheming dragon living deep underground and manipulating servants through the shadows to further its dark (or greedy) agenda. These servants might not even know who they are working for, if you want mystery, or intrigue, and a deep dragon fits nicely the concept (or even some rogue copper dragon turned evil).