It should just be automatic, the 18th level capstone, and basically equivalent to wish.Divine Intervention is an odd choice when they called out that "Mother May I" is something that they're trying to remove from the game. That ability (while I love the story implications) is entirely up to the DM to decide what to do with. You know, if it ever happens, which will be almost never.
I'm afraid that I think it needs to be excised and replaced with something more boring, but more reliable.