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.
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 and the rest of his brethren. Solheim fell after a mysterious parasite massacred its populace. Taelpar Crag in the current Cleigne region was created as a result of the war.
Ifrit was killed during the Great War of Old, but 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." A loading screen in Final Fantasy XV: Episode Gladiolus posits that Ifrit was killed after he fought the other Astrals, and was laid down upon the Rock of Ravatogh, and the Bestiary entry for Ifrit confirms this.
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.