Invasion of the Xyz Dimension

Leo Akaba, the Professor of Duel Academy, ordered his elite vanguard squad to advance into the Xyz Dimension and attack, as part of his plan to eventually unite the Four Dimensions. Without warning, they did so, causing chaos with powerful monsters like "Chaos Ancient Gear Giant" and sealing nearly all of the inhabitants of Heartland City in cards in the process.[1] Yuto, Shay, Lulu and a few others were the only survivors of the attack including, although Lulu was captured some time later.[2]

In the aftermath, Barrett was sent to the ruined Heartland City on a mission with a few of his own comrades, but they were attacked by a "Widespread Ruin" Trap Card.[3] At some other point, a Synchro Dimension Duelist named Yugo appeared in Heartland, and was challenged by Yuto. Their dragons fought each other until Yuto's allies appeared, at which point Yugo retreated.[2]

Seeking to avenge their comrades, Yuto and Shay traveled to the Standard Dimension and began attacking the members of Leo Institute of Dueling that used Fusion, sealing them into cards. Yuto attacked Sylvio in defense of Zuzu Boyle before learning that most students of LID were oblivious to the situation,[4] while Shay attacked LID members indiscriminately, including Marco and Herk, sealing them into cards.[5][2] Shay's attacks only stopped when he came face to face with Declan, the person he had wanted to lure out the entire time, after a fight with Julia, Dipper, and Kit, where the former was looking to avenge her teacher.[6]

Shay was intent on defeating Declan and holding him hostage so that Leo would release Lulu. However, Declan told him that his plan was in vain as Declan mentioned Leo would not be interested in rescuing Declan, and instead proposed an alliance as they were fighting the same enemy, which Shay accepted.[7][8]