Magic is actually Sufficiently Advanced Technology (possibly some sort of nanotech)
So the various sources of magic all came from the sky and the description of Thera getting hurt could describe a spaceship making a bad reentry. Add in the descriptions of the chamber where the Heart of the Mountain is kept and it sounds a lot like an old high-tech base (magical permanent lights, a map that had lights for each city, etc).
Given all of that it seems feasible that magic is in fact just the remnants of Sufficiently Advanced Technology. In that situation it also seems likely that it's some sort of nanotech. The Black Steel is actually just a special alloy that contains nanotech while demons are some sort of atrifically created life form that is sustained by nanotech.
When wizards use magic they basically drain the nanobots out of the steel or demon flesh and employ them for whatever purpose they need (which generally results in the nanobots burning out).
The Heart of the Mountain is actually a dispenser of specially programmed medical nanites, when an initiate touches it some of the nanites are transferred to them both to modify their body and to give them a supply of nanites that they can use for their abilities. It seems likely that these nanites are capable of limited self-replication since Protectors don't need to regularly visit the Heart to recharge although since the Heart itself is draining presumably the initial nanites are in more limited supply.
Black Steel swords can only be wielded by someone descended from Ramrowan
Black Steel swords use criteria that no one can determine to decide who is worthy of wielding them. However, part of the test seems to involve drawing blood since it mentions them "biting" the hand. One possibility is that they are drawing blood in order to do a DNA test and see if the person carries specific genes that indicate that they are descended from Ramrowan. While the casteless are the official descendants of Ramrowan it's plausible that some members of other castes are as well either due to original descendants who survived the purges and hid their identities or due to small numbers of casteless managing to successfully fake membership in other castes. That would also explain why the swords are often passed down family lines.
