The Avatar, like Ami, is originally from Earth.In the original Dungeon Keeper, the Avatar is an Expy of the Player Character of Ultima. One of the more noteworthy traits of this particular Featureless Protagonist is that he hails from Earth; it's not beyond the realm of possibility that Amadeus shares this trait. The single largest piece of evidence suggesting this in-story, though, is that he sent Mercury a horseshoe magnet to explain that he'd figured out how she captured him; he wasn't seriously suggesting she had used such a magnet; rather, it's the shorthand visual cue for magnets that anyone exposed to Earth's mass media would immediately grasp.
Adamantine is just like dwarf fortress.It guards the way to hell. This is based on a comment that Adamantineis used to denying the dark gods access to the world. At some point in the story, some one is going to dig too deep and unleash hidden fun stuff on the world. Which Ami will then defeat, with SCIENCE.
Metallia is a powerful keeper who angered the dark godsThe two known things that can help such a keeper escape the darkness are life energy, and talking to a crystal dungeon heart they control. Life energy and a particular crystal are the two ways Metallia can be freed. In this case Metallia has found a way to protect herself from the dark gods, and influence the world beyond, in the mean time, likely by reverse-engineering some of the dark gods' powers.