As the name states, this is the unofficial start of the Blood War.
This wasn't sparked due to a disagreement, but it was because the Supreme Divine thought that the demon race should be rid of throughout the whole world. He believed that they were a curse to this world, and that it was his job to lift it off of the Earth. So, a week after inheriting his title, he sent out a large mass of troops to attack the heart of it, the Demon Realm.
During that same day, a demon of the Legions had been out with his two children, (12 y.o.) Rei and (17 y.o.) Tori, had been outside practicing night combat in the forest that the Divine force was going to fly over. When a scout flew directly overhead, they all felt a breeze blow past them suddenly. Tori then flew up slightly above the trees and saw the army coming towards them like a wall, luckily, it was noticed well ahead of time before they would reach the entrance to LowGround, so, as the swiftest flier of the three, Rei was ordered ahead by his father to find someone of high ranking to warn them of the divine's sudden night attack.
Rei made it ahead scout that passed over him earlier, they of which immediately tried to attack him, but the demon broke their wing with a strong strike of his wooden practice sword to put them out of commission before diving into LowGround barely a minute later. There, he found Komanderi Kiro with his apprentice with quick timing. After laying the warning onto them, Rei passed out from exhaustion and stress. The Nine Komanderi rounded up the high ranked army demons and created a messy defense at MidGround with the little time they had, and they managed to barely fend off the large wave of divines, but several lives were lost on the Demon's side, and the injured overwhelmed those with medical talent throughout the next few days.
The Demon Lord took this act as a declaration of war, as the Supreme Divine wanted, and this led to decades of constant death.