I'm currently working on my next adventure for Middle-Earth, which takes place inside the Lonely Mountain less than a year after the death of Smaug. The Lonely Mountain is a large place, and not every passage has been explored. So, what potential adversaries and dangers might lurk inside the Mountain? I'm currently fishing for ideas, so anything you can suggest would help.

Let's see...

Maybe a bit too crazy, but you might like it:

A dwarf-child escaped the slaughter when Smaug descended on Erebor by fleeing into the lowest halls. He has survived in the darkness, Gollum-like, learning to hide his scent from the dragon, plotting his vengeance for centuries and losing his wits in the process. Now that the Black Arrow has robbed him of his revenge, he schemes against the dwarf newcomers, as he is the real King under the Mountain, of course. For his plan, think about the Phantom of the Opera. He might have a lesser ring to help him with his schemes.

Francesco

Great idea, Francesco.
Also, how about dragon eggs? As the party approaches, they start to open.
Garrett

Both ideas are interesting. The first may be a bit problematic because I plan to have an intrigue subplot for the first half of my adventure, and I was hoping to switch gears to something nonhuman for the second half. As for the dragon eggs...wouldn't that require a female dragon?

That said, having some drakes in the lower halls would work. Smaug would have killed any that he could find, but he wasn't small enough to reach all of their hiding places.

I think Francesco's Phantom Dwarf would be a great antagonist for an intrigue plot - subtly manipulating events and building a sense of menace under the Mountain.

Running some ideas up the flagpole:
Has PhantomDwarf (name long-forgotten, considers himself Durinul, (KH: son of Durin) gained control of a courtier, twisting events and communications at court (& beyond!)?
Perchance Durinul discovered the long-lost secret submerged entrance to the Erebor Deeps (connected to Dwarrowpool Cistern in Dale) and is using it to further his plots?
[see Tensen01's excellent thread My Map of Dale]

The second part of your plot could involve some dread creature that Durinul has teamed with, or is himself being manipulated by, from the Deeps?

Personally - I'd be leery of having more dragons under the Mountain - it's been done so well already, and so recently! .... whatever it is this Deeps denizen now finds the surface tunnels more attractive in the absence of Smaug. Mayhaps it drools over the presence of the Arkenstone, or requires it for some dark ritual??
Could it be a corrupted Maia striving to rekindle the long-extinct earthfire deep below Erebor!