This story is a sequel to The Conversion Bureau: The Warrior's Way
After the events of The Conversion Bureau: The Warrior's Way it seemed like life in Equestria was going to return to semi-normal. Relations between the four worlds was steady and growing. But when the Six are sent into a completely new Multiverse, things take a turn for the dangerous, as they must contend with a fourth Equestria, and an Earth suspicious of outsiders. And waiting in the wings is an enemy from the ancient past of the world on which the Six live....
A crossover sequel to both The Conversion Bureau: The Warrior's Way and Starman Ghost's critically acclaimed fic Not Alone (done with permission from Starman himself), the ponies of four worlds will learn that the legends of the past will be reborn as the enemies of all that currently seek to live in peace. Only the power of heroes past and present can save the future.....
Oddly, this fic has been mentioned on TVTropes.org, but has no page of it's own. Ah well. One can only hope.