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For those who are yet to complete the game this page may or may not contain spoilers regarding the plot. Discretion is advised.
Experiment Zero, also known as the final experiment was the final part of a series of experiments carried out by the folowers of Aios and the Vestra within the Towers over 500 years prior to the game's story, during the War of Unification. The Elders of Aios had made a deal with the Vestra to create the 12 Masters in exchange for the Vestra being given their own homeland. This was later seen upon as being over-reliant on the Vestra by those of the Elyrian army attempting to repeat the experiments 460 year later.
Experiement Zero was specifically to create the final Master, intended to unite the 12 gods of Aios and thus unite the realm. To do this two vessels were required (unlike the one vessel requirement of the previous 12 Masters), these vessels would be a man and a woman to represent a balance between the 6 elements of Aios.
The Complication and CurseEdit
However mid-way through the experiment, it was discovered that the Woman was pregnant with the Man's child, causing an imbalance which meant the experiment had to stop. It was too late for the woman to be saved however, turning her into a final "incomplete" master. Her child was safely delivered with the aid of the Oraclos Chain and was raised by her husband (who was also saved), this child is later revealed to have become Aeron's ancestor. With this the towers were sealed along with the original 12 Masters.
Zeron, the woman, distraught from the loss of their first born Ruben (which caused the couple to volunteer as vessels) and now her remaining family, decided to place a curse on the lovers of her husband's descendants. This curse is what lead to Elena's transformation and the events of the Harvest Festival just prior to the games beginning.
460 Years Later...Edit
During the War of Independence the Elyrian army had found texts from the previous experiments. Intending to create a set of living weapons in the Masters, the army had contacted the remaining Vestra to ask for their assistance, which lead to the events of the Final Experiment and the Catacylsm.