The First War was a civil war between two factions of immortals.
As with many things from ancient times, the exact trigger of this war has been lost. Though it is suggested all of the fighting stemmed when one of the ancient cardinal laws was defied by a high ranking soldier. He was ceremoniously executed, resulting in a massive revolt by those that disagreed with the ruling and the law itself.
Though conflicts were not uncommon, the scope and devastation of the war had never been seen before, and the madness would echo for eons to come. Eventually a side was deemed victorious, and the remains of the opposition were exiled to a new enclosed sector that had formed in the chaos. This broken world became Hell, the prison world of the fallen angels, who would evolve over time into what became known as Demons. Those that stayed in the Kingdom of Heaven would similarly become modern Angels.
This would technically be the end of the First War, but the conflict would remain and reemerge when Angels and Demons eventually came back into contact with each other. Though there have been times of peace and ceasefire, the war would never truly end.