The flames of war surge as Solheim fends off the Pyreburner’s fire. The gods' pleas for peace fall on deaf ears, and the battle rages on.
Great War of Old (神々の「魔大戦」, Kamigami no 'ma taisen'?, lit. War of the Gods), also known as the Astral War, was a war two thousand years ago in the Final Fantasy XV Universe that led to the collapse of the ancient civilization of Solheim. Not much is known about the war at current times.
Long ago, the traitorous Ifrit waged war against the rest of the Hexatheon. Many moons passed as the embittered battle raged on, culminating in a clash between Bahamut and Ifrit in which the latter was laid asunder atop the Rock of Ravatogh. Taelpar Crag, the grand gorge dividing Duscae and Cleigne, exemplifies the severity of the War of the Astrals while also symbolizing the rift that separated the Six.
Solheim was a nation that prospered during the time Eos was stable under the watchful eyes of the Six, the Astrals. The war was started by the Astral of fire, Ifrit, who ruled over Solheim but was betrayed by his once-loyal subjects. Ifrit sought to destroy Solheim as punishment for their betrayal, but as the Six are sworn to protect Eos from all harm, even from each other, the other five turned on Ifrit. Solheim fell after a mysterious parasite massacred its populace. Taelpar Crag in the current Cleigne region was created as a result of the Astral War.
Ifrit was killed during the Great War of Old, and his body was laid upon the Rock of Ravatogh. The other Astrals—bar Bahamut—fell into slumber, as the world was left in ruin. Ifrit was resurrected by the Starscourge. When Ardyn Izunia ushers Ifrit upon Noctis Lucis Caelum in Insomnia two thousand years after the war, the latter summons Shiva, the Astral of ice, who says: "Pyreburner. That heart of flame was turned to ash once. A dead flame must burn no more. Taste again the chill wind of death."
Two thousand years ago is the time when Astrals forged covenants with a man who would become the king of Lucis, but their revelations are said to have left devastation in their wake. This seems to coincide with the Great War of Old, as the king is said to have fought darkness alongside the Astrals at this time, and "the plague" is said to have subsided by his hand. The Astrals chose two mortal bloodlines to gift special powers to fight the darkness. Afterward there was a long period of stability and peace, during which four new nations sprung up, yet the plague would later be re-introduced to the world when the Niflheim Empire began experimenting with daemons.