We could use the very story of D2 for source material if we wanna get theoretical Everyone lost their light, our guardian found shards of the traveler to restore our light and the traveler reshaped our powers to be what we needed to defeat Ghaul. im no Byf, nowhere close, BUT i imagine that is the best lore related answer you could get And mind you, this is ALL speculative, but it just makes the most sense imo.