The 5 headed dragon is a "visage", just a minifestation of of a gods precence. In game terms, a minfestation is template that can be added to any creature. (It's in the "Holy Orders of the Stars manual) They are defeatable, but as you said, they would take an army to defeat. lol. But they are defeatable, and I'd use'em sparingly (manifestations) anyway. The gods don't usually take a direct hand in things, but they will definately have an impact on the story.