ok let me explaint it in FFXIV terms
Altana = Hydaelyn
Promathia = Zodiark
Emptiness = thats hard to explain with a few words i will quote some1 because kinda explains it
Promathia isn't really the God of the emptiness, the emptiness is its own force that started consuming him... Altana sealed bits of emptiness inside every person to keep it from building up enough to completely consume Promathia. The mother crystals funnel bits of emptiness into souls. The Zilart who draw the short straw and have emptiness put in them lose their powers and become Kuluu. The other Zilart freak and start trying to open the Gate of the Gods to escape that fate. But when the Zilart start using the mother crystals as batteries, the crystals can't process the emptiness fast enough and it starts building up. The built up emptiness eventually sucks up AlTaieu. No more Zilart, so Altana creates the five races to be the new Emptiness containers.
Selhteus, like all the other idiot genius Zilart, thinks this just means "Emptiness = Bad" instead of "Zilart messed up the Emptiness containment system" and makes a pact with Bahamut to destroy the emptiness altogether. Which means destroying mortals that house the emptiness. So Bahamut's rampage is another Zilart screwup.