History of the world

I have some vague suggestions.

As we said there should be remnants of a civilisation around, but no definate answer as to what happened to them.

i see the dark lot being focussed on redstone, believing it to hold great potential. It could be their belief that the old civilisation was wiped out due to not being able to control the power of the redstone.
the dark could want to get as much restone as poss and harness its power and technology. Kind of like the brotherhood of steel but more 'bad'.

The light lot could obviously be concenred over the use of redstone and want to contain it and use it sensibly.

Or some such

I'm also thinking that, whenever the descendants of our lot first arrived they found massive wealth in the mines and hence managed to get cities up pretty quickly but also caused the division of the two (or more) factions.

Sounds good!

We could say something like they found this crazy material (maybe obsidian or the blue one or whatevs?) but argued over it and had some big war, which broke the stone in two.

Each side stole off with half and used it to build their cities before the power diminished from it (like some magical jazz) - now each faction could have a "monument" made of the stuff (like a sculpture) which does nothing in-game but it like a status symbol for their kingdom