anyway.

Principles for prep?

That's pure gold. Thank You. However:

"No one but the PCs will undertake this" - Can I ask why? Doesn't this rule out the possibility of the dramatic race against someone undeserving or untrustworthy who might get there first...

If it does, it shouldn't! Very good catch.

Isn't a prophecy just a special case of opportunity?

Prophecy: "When the stars align, old evil will stir and tear the world asunder."

Opportunity:
Cultist I: "Oh look, the stars are aligning."
Cultist II: "Cool, I guess that's a golden opportunity to make old evil stir and finally tear the world asunder."

In other words, I don't quite get it.

Not to speak for Vincent, but I think the "opportunity vs prophecy" thing is meant to avoid pre-scripted events. A prophecy is a magic thing that's going to come true in the future—a set-piece already positioned up ahead that the game has to navigate its way towards, which puts a constraint on the game just going wherever it goes. "An old evil will stir and tear the world asunder"—if it's actually a prophecy, then that's will tear the world asunder, i.e., we know the world gets torn asunder, that's one thing we already know. If there's a hidden "unless of course the PCs stop it", then that's a weak sort of prophecy—it's just a threat with a countdown, but not a magic "thing that will come true". (But the quest isn't just to avert that happening; it's to improve the world, not to preserve it.)

If the players are the cultists and tearing the world asunder is their version of improving it, that's cool, maybe they could have an opportunity to do that. But insofar as "old evil will tear the world asunder" is a prophecy, that's still not an opportunity; rather, their success is assured. Prophecies that turn out not to come to pass after all are false prophecies, no better than horoscopes.

Even if the prophecy isn't predicting the success of tearing the world asunder, even if it's "when the stars align, the Grave Throne will appear, and he who sits upon it may tear the world asunder", giving your cultists an opportunity (sit on the throne), still the interesting part isn't the prophecy's prediction. Don't spend game time on whether or not the Throne appears or not if we already KNOW it's going to appear; just have it appear already and go from there.

Am I guessing right here?

@Whirlmeister: You might read it like that, but you don't have to. I read it as, "no one but the PCs will undertake" the "dramatic race against someone undeserving or untrustworthy". In other words, if that someone exists, then the race itself is the opportunity, and the someone is the enemy, whose reach is long and whose allies are powerful. :)