I played a little further, and got the curious result of conquering a useless Barb city, which of all things CAN build Lightbringers. I figured maybe lack of religions/temples could have something to do with it, so I stuck a temple of Empy there, it didn't stop the LB being an eligible build. See Sludgehome, a little yellow dot in the midst of some overflowing Sheaim culture.
Getting excited about it, I settle Belerian in the extreme North of my Malakim lands, but oddly, no dice, LB isn't available.
Excuse the sloppiness of the save, this was a bit of a waste of a game, everyone in their own mountainy strongholds isolated (except I think the dwarves and Calabim have a tiny land bridge which facilitates war), so I have a lazy no-military goof off kind of game here, just checking out new features. Again, I guess it's not civics, absence/presence of religions, I'm just plain confused as to why one spot can build LB, the rest can't. *shrugs* I must've been able to build LB within the last hundred turns, always fun to do the "March" Disciple of Leaves up the ranger/beastmaster line.