I am running the final part of my norsca WFRP adventure tonight and there is a bit too little left of the adventure to really last the evening, so I am trying to come up with some good ideas for how to run it and possibly fluff it out a bit.

SETUP:
The characters are tasked with returning the sword of Thaugnir Wurmabrekkir to his tomb, which is at the entrance to the epic frost dragon Urgablaudinggars lair.
The lair is actually an ancient jotun mountian city (think Mines Of Moria built by giants instead of dwarfs). The city fell in an attack by deamons and mutants before humans walked the lands.
The characters have fought and climbed their way up the mountain in a blizzard. Towards the top, they were attacked by the dragon and fled into a cave that turned out to be an enterance to the jotun burial halls. Finding their way through the catacombs, avoiding the chaos spawn that haunts its halls, they come to a set of stairs leading upwards with freezing fog gently rolling down from above.

So, the basic plan here is that the characters have entered the lair from the back entrance and need to get through the jotun city, past the dragon and to the large entrance at the other end.
The dragon is basically invincible. The characters have no way to kill it and probably no way to even hurt it. The dragon can wipe the party in a couple of rounds (something I have allready made clear when they got attacked on the moutnainside and the dragon ate their companions).
The only way to defeat the dragon and win (ie "not die"), is to get the sword to Thaugnirs corpse. If the sword is put into the hands of Thaugnirs corpse, he will rise and defeat the dragon, forcing it back into the depths of the mountain, and the characters will have completed their quest and saved their village from becoming dragon chow.

What I have been thinking, is having the dragon slumbering and then being awakened by the characters and hunting them through the ruins of the city.
This is a bit short for an evening though. So I have tried to come up with some extra ideas. The only thing sort of substantial I have is this:

A cult of mutants who worship the dragon as a god. They might try to capture the characters and sacrifice them to the dragon. Or they might be possible to actually talk and barter with. They might even have a "quest" for the characters.

So, does anyone have any suggestions for what kind of problems I can throw at my characters while trying to sneak past a dragon in an ancient mountain city?

Well, you seem like you have a wealth of options which the background you already have!

The cultists sound like an interesting idea. If you want them to be supporters of the Dragon, then the players are either going to fight them or deceive them in to thinking they are friendly. Perhaps there is some chaotic device deep within the mountain that prevents the Dragon from achieving full power? The cultists are trying to seek it out, which would give you the potential of a mini-quest.

I would find it interesting if the cultists are actually enemies of the Dragon, perhaps chaos or undead maybe. This could give the opportunity for an 'enemy of my enemy' situation. If you balance the difficulty of the quest so the players think they may not be able to complete it on their own (beset by opposing forces), maybe the party is prepared to team up to defeat the dragon. Obviously there is the aftermath the players have to consider because their Dragon defeating assistants may turn on them.

Or perhaps they encounter group of Dragon Slayers encased in magical ice. How do they break the spell? Or if you prefer a little ambiguity, maybe if the warriors encased in the ice are human and their allegiance is unclear.

One last idea is for some magical traps set by either the ancient Giants or the chaos to protect the halls could provide some side adventures?

Hope this helps,

Khelrauko

Hm, interesting.

Ok, combining the cult ideas:

There was a group of mutants living in the city. Then the dragon awoke and threw evervything into chaos. The mutants split into two tribes, the ones that worship it and the ones who avoid and fear it. The characters run into the guys who fears it first. They recognize the sword they are carrying. They show the characters cave paintings of how Thaugnir guarded them until his sword was stolen, the dragon rose and their people split in two. The characters will be able to ally with the mutants and with their help as guides sneak past the dragon. Then they run into the dragonworshippers, a fight starts, which wakes the dragon. Cue desperate cat-and-mouse game with the dragon through the ruins until they reach Thaugnir.

You can also have another expedition present, an arrogant scholar/wizard spouting pseudo-history of the area or dwarf slayer group etc. A group that complicates things, pokes where it shouldn't, perhaps is really cultists, covets the tomb and would loot it or otherwise "screw things up", could also "distract dragon" (for a few moments).