Figured I’d get this out there while I can still post before my sub expires.
From Chronicle 1, right at the start of the book:
The cataclysmic birth of the cosmos also flung shards of Light throughout reality. These shards suffused the matter of myriad worlds with the spark of life, giving rise to creatures of wondrous and terrible diversity.
That would explain it, though I think that takes the “shard” part very literally.
Isn’t ‘Sharding’ how the servers function?
My theory is that it’s part of a massive “Greater” Naaru, like a Lightlord or somesuch, that somehow got destroyed in The Great Dark and had its pieces fall away with one of them landing on Azeroth. We see it as a Void crystal at some point, and Naaru can have a Void form.
Since the zone is under-ground, I had a strong speculation it was ‘the tip’ of the sword Sargeras stabbed into Silithus, glowing radiantly due to the Azerite it pieced through.
However I wouldn’t mind if it was one of the shards of light mentioned in Chronicles. 
I feel the head-nods to lore that were told in the past are pretty great for the story & can give it good fluent world building & writing (way better than Shadowlands at least).