Cai returns and has fully recovered from the events of Generation 10 and has resigned his position as Royal Alchemist. By the order of Sinead, the Milletian is required to bring Cai back and is to return the Alchemist Medal to him, something given to all Royal Alchemists. The Milletian then goes to Tara's Stonehenge in order to search for Cai inside Shadow Tara Rath Castle.
After the Milletian finds Cai, he notices the Alchemist Medal, and states that he has no intention of becoming a Royal Alchemist for a while. He explains that he is part of the reason for the threat on Erinn. However, he does take the Alchemist Medal back since the Milletian went through the trouble of finding him, but can not make any promises of returning as a Royal Alchemist. Cai then says he will make sure Caliburn returns to the Castle, but requires Leymore's help, and sends the Milletian to find him. He will return the favor for searching for him and returning his Alchemist Medal.
After talking to Sinead about Leymore's whereabouts, the Milletian discovers that Leymore is in Shadow Tara Rath Castle in search for traces of Caliburn. Upon arriving, the Milletian sees Cai with Leymore. Cai then thanks the Milletian for tracking Leymore down so he could get in touch with him. With the Milletian helping the two alchemists, Cai states he feels like an army is helping them. After some investigation the Goddess Morrighan appears before Cai, Leymore, and the Milletian. She is aware that they are searching for the Caliburn, and informs them about its history.
Cai is summoned by Sinead, along with Leymore, to help the Milletian assist Buchanan, who is currently investigating in Shadow Tara Rath Castle. Cai is under the assumption that Buchanan is in danger. Although he is investigating clues about the Parthalonians, Cai hopes that Buchanan is not in harm's way. He is impressed that Buchanan can do research even in the most dangerous of places. After searching the entire first floor of the castle, the group find Buchanan unharmed. He explains what he has discovered and asks them to obtain the Book of Partholon. Buchanan then says that the book is hidden by a secret mechanism found in the castle's Restaurant. He then hands group a crudely drawn map with the correct switch order. After leaving the Shadow Realm, the group then reports to Sinead and return to the Shadow Tara Rath Castle to find the Book of Partholon. Cai doubtful about the book's location, worrying about the Fomors moving the book.
Upon arriving at the book's location, the group was ambushed by Mata and he summons Shadow Monsters in an attempt to stop them. Cai attempts to persuade Mata by saying Cichol warned them about the danger Erinn may face and that it could affect both Humans and Fomors alike. Mata replies that Cichol is no longer their king and is a coward who has forsaken the Fomor Ideal. Mata attempts to attack the group, but is suddenly stopped by the Python Knight, in the name of the Holy Land. The Python Knight proceeds by eliminating the Shadow Monsters, along with Mata, with Shadow Death. Cai demands that the mysterious Python Knight reveal his identity, exclaiming he is neither Human nor Fomor, but the question is evaded. Cai believes the the Python Knight is bad news, questioning who he is. He will attempt to research the Python Knight and find whatever he can on him. He instructs Leymore to stay with the Milletian and investigate the Parthalonian magic, leaving shortly after.
As Leymore searched for a way to recreate the ancient Parthalonian magic and the Milletian restored the Book of Partholon, Cai underwent his own research on the Python Knight. He discovered that the Caliburn Altar is protected by a barrier of ancient Parthalonian magic. Unable to do anything alone, he sends word to Padan to give the information to the Milletian. The Milletian then meets with Cai on the outskirts of Shadow Tara. Cai explains that while researching the mysterious Python Knight, he had found a connection between him and the ancient Parthalonians. The Parthalonians carried a key that would open Partholon's Barrier, which surrounds and protects the Altar of Caliburn. He then points down a hill, calling it the Caliburn Pass. The Partholon forces had gathered in a spot that would stop Cai alone. He asks if the Milletian and they agree to help Cai get to the Caliburn Pass. After obtaining the pass, he goes to find Jenna and Leymore.
Leymore, Jenna, and Cai arrive at the Altar of Caliburn. Leymore and the others had trouble with Partholon's Barrier. Upon their arrival, the group finds the Milletian surrounded by the Python Knight's minions and offers to take care of them while the Milletian takes care of the Altar of Caliburn, returning Brionac to its rightful resting place. By doing so, the Milletian removed all the Parthalonians, including the Python Knight, from the area around the altar. Jenna begins to think about how it was all started over the Tuan's selfishness, and how it affected his thinking. Cai finds that the Shadow Realm still exists, even though the Caliburn is now in its rightful resting place. Leymore suggests that the Shadow Realm will continue to exist and it is something that can not be undone.