Since well before the war began, one Corusa Industries laboratory on the moon had been laboring to produce what would prove to change the world. Two months into the conflict, a cruiser named the Arbalest was sent into space to pick up and deliver a piece of cargo from the lunar base. Upon delivery, the crew was surprised to discover that the cargo they were to transport was, in fact, an AT - the next generation of the Kerykeion. Both the trip to and the return from the base were fraught with danger, as the ship was repeatedly attacked by enemy forces stationed throughout the area - it was in one such battle that the Kerykeion Mark II belonging to Alexander Kanbara was critically damaged, forcing him to return to the ship. Under the threat of continued attack, Alex chose to launch in the prototype unit.
Even incomplete, the Mark III was a deadly weapon on the battlefield, with its beam submachine guns strengthened to the point where Peskas could be taken down with ease. It lacked an armblade in its incomplete state, but the pilot compensated for this in its initial fight by using the frame itself as a weapon.
It was soon discovered that the reason for the Mark III's power was also its greatest danger - the unit used a new kind of reactor, the Baryonic Drive, that provided effectively unlimited fuel and a very high power output. However, the reactor's output was higher than the frame could handle in its current state, causing the unit to release the excess in the form of a blue-white mist from the joints. Though visually impressive, this effect was indicative of extreme strain on the Mark III, and a few engagements with the unit demonstrated that unless it burned power very quickly, it could easily overload the frame and break down. As a stopgap measure, one of the mechanics onboard designed a set of equipment to slow the Mark III's rate of overload consisting of a pair of sword rifles and a large blade on the shoulder nicknamed the Buster Sword.
The sword/rifles, normally stored at the waist, helped compensate for the lack of an armblade as well as burn power - in rifle mode, they provided extremely powerful shots, and in sword mode served as solid close-range weapons. The Buster Sword was normally kept docked on the shoulder, where it stored up excess energy and used it in the formation of a defensive globe barrier around the Mark III. Though not perfect, this version of the Mark III was able to handle itself quite well in combat, and proved unbeatable until it came face-to-face with a new model of XM, the Korol. Devastating the Mark III with its superior firepower and graviton axe, Korol's pilot made short work of the Mark III, and would have destroyed it but for the arrival of reinforcements to their battle.
When the critically damaged Mark III was rebuilt, its frame was completed and strengthened, allowing it to handle its reactor output. Twin shield harmonizers, miniaturized versions of the shield generator from the Buster Sword, were mounted on the hips, and a flight pack was mounted on the unit's back. Most importantly, the Mark III was given its armblade, an improved model that was easier to handle than the Mark II's. In addition to this, it carried a physical shield on the left arm that provided a more effective defense against the beam weapons employed by most enemy machines, and standardized hand plugs allowed it to wield weapons designed for the Mark II.
The Mark III would be used by Alex until near the end of the war, when it was mass-produced in its completed form. In that time, it would prove itself superior to nearly any XM, with the exception of the one who would become Alex's rival...