There's a decent chance you already know this, but that bit is probably a reference to Harry Potter and the Wastelands of Time. I don't think that fic describes what did its Atlantis in in any great detail (though it's been a year or two since I read it), but whatever it was, it was probably an apocalypse of subtype dug too deep and awakened an ancient evil. All the absurdly lethal skeletal guardians left lying around seem to point in that direction, at least.
On the other hand, there's enough references to D&D magic floating around that balefire wouldn't be too outré.
(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: