Phoenix War

The Phoenix War is the term used to describe the series of conflicts between the Gotei 13 and the Inboyuigon. The Phoenix War earned its name due to being originally centered on the earlier conflicts of the war, which revolved around the Inboyuigon's infamous theft of Zanpakutō, the Sōkyoku. Currently, the Phoenix War, in general, refers to the war between the Shinigami and the Undouka. Chronologically, the Phoenix War has been fought by both sides for over 10,000 years, and is currently the longest standing war fought by the Gotei 13.

Origins
Some several decades after the Spirit King's royal guard discovered the Vale of the Shinraifu, only a small fraction of the guard became enthralled in discovering it's secrets. After the lure of powerful arts began seducing the more curious members of the guard, the Spirit King's royal council saw a potential threat, and exiled the members of the royal guard who refused to abandon the pursuit of the "angels". By that time, however, approximately one third of the royal guard were devoted to the temptation of hidden powers. Exiled from the realm of the Spirit King, and banished from most of Soul Society, centuries later, the deviants discovered a way to enter Kongoukai, and resettled in the pocket dimension for a nearly a millennium. Most in the Spirit King's council believed that with their banishment, it was the end of the obsessions with the Valley of the Angels. The though of an eventual reemergence of the banished Shinigami was inconceivable. Unknown to residents of the Spirit King's realm, was that the exiled Shinigami discovered a way to travel back to the Spirit King's realm, and continue uncovering the secrets within the ruins in the Vale of the Shinraifu.

Zero Hour
The emergence of the Inboyuigon came some two thousand years after their banishment. With the aid of the Spellcasters, the Undouka were able to merge the dimensional planes between Kongoukai and Soul Society, with the focal point being the center of the Seireitei. With a fighting force of over 5,000 Undouka warriors, including ten Guardians and three Councilors, the Inboyuigon launched a major assault on the Gotei 13, both revealing their existence, and making the first strike in the war that followed their move.

First contact
With the element of surprise, the chaos instilled in the unprepared Shinigami, and the use of exotic spells, weapons, and techniques, the Undouka rained down from a merged point in the sky. With the Shinigami paniced, the Undouka had effectively broken the back of the Gotei 13, killing several captains and lieutenants, and holding their own against the additional corps. The first contact between the Undouka and the Shinigami marked what many believed to be the true beginning of the Phoenix War.