My current best guess is that it's Zodiark / the Ascians.
The Echo, the "power to overcome," allows us to overcome Zodiark's curse and rewrite fate. The Ascians also have it, but use it to the opposite end - keeping fate on course. There seems to be a big problem with Hydaelyn's actions; she seems to be violating causality and fate by allowing our existence, while we seem to be the only one the Ascians pay any real attention to. Ol' Elidibus also seems intent on getting Urianger to buy into his "this was all written" philosophy, but the latter is intent on fighting "our fate to perish."
Well, time will tell the big answers.