Ogremoch certainly doesn't need to go far. But he has to have some wants/desires/goals. After all, he's probably been thinking about what he would do if the prophet actually succeeded. So, I guess the question is, what would he have planned to do?
The earthquakes, the random portals with elementals appearing, all that happens regardless of where he goes.
Note, even though the characters are typical murder hobos, they are still good aligned and not evil, nor really tending that way.
I'm also thinking of bringing Entemoch into the story line. Something along the lines that he loves his brother, but can no longer stand by passively while his brother does evil on others. To continue to do so, would mean he would be complicit in evil and that in itself would be an evil act. So, now that his brother is upon the prime material plane (and it therefore more vulnerable than normal), he will help the adventurers kill his brother by providing them with something (I'm thinking arrows to neutralize the burrow/earth melding abilities to keep his brother from escaping).
I had thought about Sunnis and Uzrith, but don't see a need to involve them in the storyline.