This story is a sequel to Flash Sentry Chronicles: Rainbow Rocks
Flash, Twilight and friends have beaten the Sirens. All seemed well as they returned to Equestria, but true peace never lasts. Now, the Elements of Harmony find themselves in a place that will reveal a truth that was thought to be all but forgotten. And as a darkness rises from these depths, will they survive this revelation, or is it all over?
Written by KingJoltik
Now on TVtropes