Zodiark longeth to be made whole. For His restoration, for His resurrection, His servants labor without cease. They seek to tear down the barriers which surround the Source. Thus do they rejoice in their Ardor─in your calamities─for each marks a Rejoining. Seven times have they succeeded. Seven times hath the Darkness grown stronger. Seven times have I failed.
The Rejoining (also called Ardor or Calamity) is a recurring phenomenon in Final Fantasy XIV. Initiated by the Ascians, the Rejoining is an attempt to restore their god Zodiark's power, an act that is accompanied by cataclysmic turmoil that almost always destroys whole civilizations in the process. These events are known as Umbral Eras to Eorzean historians, with the term Astral Eras referring to the periods of prosperity in-between these calamities.
Long ago in the aetherial sea, Light and Darkness once existed as a single entity. But the Darkness, Zodiark, coveted power. The Light, Hyadelyn, was forced to banish Zodiark into the heavens as a moon bound, forever separated. This sundering also shattered the barriers between planes, and the world became split into thirteen pieces: each reflections of the Source.
Zodiark longs to become whole, and for this end, his servants, the Ascians, continually sought to shatter the barriers surrounding the Source. To achieve these Rejoinings, a chaotic confluence of untold proportions is required, all with disastrous effects on civilization. Even the technologically advanced Allagan civilization of the Third Astral Era was utterly shattered by the fourth Rejoining. The only known subversion was during the Seventh Rejoining, when the primal Phoenix, fueled by prayers for a realm reborn, destroyed Bahamut midway through his apocalyptic rampage.
The periodicity of these calamities was noted by Sharlayan scholars, who developed a historical dating system based on the cycles of prosperity and ruin. These Umbral Eras were noted to be preceded by the sounding of the Echo. A group of scholars known as the Students of Baldesion of the Isle of Val sought to study the Echo and its links to the Rejoining phenomenon. Sometime after the seventh Rejoining, the isle was destroyed under mysterious circumstances, with the only known survivor to be Krile.