The Adenian-Tethish War was the deadliest war in modern history, claiming over 90,000,000 lives, and occurring from 504 GD to 515 GD. It was the last war of the Adenian Expansion, and marks the only failed attempt at expansion by Aden under Kir Gendu XXIII, as well as the end of his reign. The war resulted in a truce between Aden and Teth forged between then-Emperor Kir Gendu XXIV and King Vazun III, with a promise of reparations in the form of labor and reconstruction aid rendered unto Teth from Aden.

Following the success of the Adenian-Opananish War, the Annexation of Kalt, and the Adenian Occupation of Risila, Kir Gendu attempted to expand overseas by invading Zis in early 504. However, an alliance of forces from Teth, Fane, and Dinefi were able to turn back this initial invasion. The king of Teth, Vazun III, sent a formal declaration of war to Aden's emperor, Kir Gendu XXIII.

One week after the declaration of War was received by Aden, the Tethish capital of Tainra was bombed from the air. A second bombing was attempted two days later, however this bombing run was largely unsuccessful due to the arrival beforehand of a batallion of mages who used pondemancy to pull the planes out of the sky, causing them to crash into the ground outside of the city. As this severely damaged farmland outside of the city and only narrowly missed crushing a farmer to death, however, the Tethish anti-air strategy was refined following this.

In the year 511, Aden used a nuclear weapon against Teth, bombing the city of Geil. This bombing destroyed the famed walled city, and made very clear to Teth that conventional military tactics could not work against this new technology.