I find light-dark oppositions like this interesting, occasionally, but they don't work well plugged into larger fantasy worlds. I think, if you want this era to work aesthetically, you need to simplify, simplify, simplify -
1) Forget about having them in the same world as Wesnoth. Have them in their own world, and have them be the only civilizations in existence in that world. If your world is about light v. darkness, your world is about light v. darkness - not about light v. darkness v. loyalists v. knalgans v. drakes v. northerners v. rebels v. undead.
2) Don't even talk about different fantasy races. Don't have them be elves, humans, orcs, dwarves, or anything. Just have the one species of rational bipeds that you call "men". If you want them to be immortal, fine, but they're still "men", because they're the only rational beings in this world.
3) Obviously the backstory would have to be changed to accommodate this (the stuff with the demonic sword, for example - it could work, I guess, but you can probably think of something better), but honestly, I don't think this is a big deal - it seems clear to me that the only thing you're really set on here is light v. darkness, and the rest of that backstory is just a not-that-well-written excuse to set up that dichotomy. So, you need to come up with a new backstory - an explanation of how the dichotomy came about, or just no backstory and have it assumed that there are these two sides, light and darkness, that are eternally fighting.
Feel free to ignore this advice, but honestly, if you ignore it, it's very doubtful your era will ever turn into something interesting, while if you follow it, there's a chance (though small) that something decent might come out of it... I'm just telling you this for your own good so you don't waste your time on something worthless. I apologize if this offends you.
