The Eve of Disaster is a tragic event that happened a year before the events of Forever Kingdom. The Forsaken Village is now deserted, and many people in Solta were killed, because of the events that took place during the Eve of Disaster.
In the middle of the war between Solta and Morea, an incident happened in which an army of monsters known as Mutants appeared "out of nowhere," according to Felkin, and attacked the Forsaken Village. They devoured the villagers alive. The Mutants were led by a young girl.
The Mutants presumably spread further to Solta and Morea, as Felkin states that both sides suffered losses. Felkin, and presumably many Soltans, lost many friends. The monsters then just vanished, like smoke. A year after these events, people fear it and avoid talking about it.
When Felkin recounts the events of the Eve of Disaster to Faeana, she gets upset and tells him not to repeat "stupid stories like that." Felkin chides her, and says that he has to seek out the truth about it, or his friends would never find peace in the afterlife.
At the Forgotten Valley, another slice of the truth is revealed: Darsul, the sorcerer whom the party had encountered at the beginning of their adventure, used the soul of Solca, one of the Twin Goddesses, to be used as ammunition for the Palmira Doll, a most powerful weapon. However, use of the Palmira Doll backfired, causing a legion of Mutants to flood the land and devour all the humans they encountered.
In the end, when Darius, Ruyan and Faeana put the souls of the Twin Goddesses, Solca and Karmyla to rest, Karmyla reveals that prior to the Eve of Disaster, Darsul sought to use Solca's power, but only sought control of her by destroying her soul. Solca's soul was ripped from her body, and bound to a little girl with a heart as pure as hers -- Faeana. It was then that Faeana understood that she was the one who led the Mutants to destroy both Solta and Morea; she was the one who caused the Eve of Disaster.
After leaving the Sanctuary where Solca's and Karmyla's souls remained, Felkin extended an invitation to Darius, Ruyan and Faeana to join the Solta army. Faeana takes leave of her friends, as joining the army would only bring sadness to her, as she caused the Eve of Disaster and killed many people in her wake.