The core concept of this adventure is the classic discovery of a seat of ancient evil - one that is not too ancient to protest at being disturbed, of course. A tough adventure that can work for low level parties (up to 6th-level is suggested, although it may still be challenging for slightly higher levels), playtesters survived in the main by running away and at least in one case by cutting a deal they will probably live to regret!
To locate this adventure, all you need is a mountain that nobody dares to climb. If running as a stand-alone game, you can either read out the legend or have the characters discover it in nearby settlements to start them off, or it can be placed in a suitable area of an existing campaign world. It's also suitable if you need the characters to quest for an item... they'll get a bit of a surprise when they go to pick it up!
The adventure proper opens with a chance at a bit of role-playing and interaction, with a daft old coot who lives half-way up the mountain and has some passably useful information if the characters have the patience and tolerance to listen. Further up comes the adventure site itself, and from there on, it is classic dungeon crawl time. Plenty to trap the unwary or unlucky, some loot to gather and more undead than you can shake a stick at! All excellent poke around at your peril stuff.
As a bonus, there's a brief stand-alone adventure, The Tower, at the end. Who could resist a treasure map and a gold key engraved with a heart? If you go there, you probably will wish you had resisted, after all.
Classic 'old school' adventuring, well-presented and atmospheric, and with a coherent underlying rationale: everything is there for a purpose.
Death Frost Doom