Well, I think it is more of a foreshadowing thing. Like starting the whole game into a more PvP oriented gameplay.
"The time has come for you and 'yours' to join the fray, Warrior of Darkness." Sounds to me like there will be several of them, all striving to battle the Warrior of Light. A bit like a two Faction System in the next expansion, where you have the light guys fighting the darkness guys. Of course both are doing the "just" or "right" thing. If it does hint at a new playable faction (mind you with most likely the same races and classes) it would make sense to take the most generic representative for one of the Warriors.
Going with usual Fantasy philosophy, total light would mean stagnation and petrification, no change, every standing still. Maybe the second moon they are standing on is like a place where the Light was successful, and that is what they try to prevent (extremes either light or dark being bad).