Alphaks the Dark desires that key, by which he plans to release death and chaos into the realms of man. Your party may be all that stands between life and death. Will you heed the lunatic ravings of a dying madman? Travel to the top of Guardian Mesa, and enter the Septahenge. Gather the mystic symbols, create the key, and defeat the Carnifex, before it's too late...
Barring the occasional piece of unimaginitive puzzles or tricks, and of course the standard bit of railroading, this one wasn't actually all that bad. A solid chunk of imaginitive landscapes and tough fights.
Continuing my Moldvay kick, I came to this, an extremely high level and deadly adventure, involving travel to various coloured realms and retrieving the artifacts from each that are required to open a portal and stop a malevolent group called the Carnifex from escaping their eternal prison and wreaking havoc on the mortal plane.
Each of the realms are unique and interesting in their own way, filled with challenges both combat and non-combat. I'd perhaps vary it up so there aren't quite as many dragons, but that's just for personal taste (you gotta be judicious with your dragon use).
Once the artifacts are obtained, it's onwards to the vast citadel on the cover, through a dungeon with traps that are, to my surprise, actually pretty funny, and then a classic and very memorable finale. If you're searching for something epic to act as a capstone to your campaign, you'd do well to give this a look.