Aegislash first appeared behind Celosia when Shauna became hypnotized by her Aegislash.
When Celosia appeared to the group, Aegislash was behind Celosia, who was behind Shauna. She explained how she came to control Shauna and that Aegislash is the one that is controlling her. As Korrina and her Mega Lucario jumped from the roof of the Santalune City Gym and charged towards Celosia and Aegislash, Aegislash was ordered to use Slash, which clashed with Mega Lucario. Korrina then ordered Power-Up Punch, with little effect. Korrina questioned Celosia on it being a ghost-type, with Celosia saying is correct. She ordered Mega Lucario to attack, which caused Aegislash to flinch. As Kanga joined the battle and Mega Evolved, Lil' Kanga knocked it backwards and caused it to change into its Shield Forme. While stuck in Viola's Surskit's Sticky Web, ordered it to use King's Shield. During the battle, Celosia told Aegislash and her Honedge that it was time they went back.
Malva borrowed Celosia's Aegislash for a test. Aegislash took control of 15 Sky Trainers, who attacked Y and the helicopter the rest of the group were flying in. Knowing Aegislash was near, X had Mega Evolve his Manectric, who used Discharge to defeat Aegislash. While Aegislash was on the ground, Malva watched the group who were in the sky. Malva reunited with Celosia and gave her Aegislash back, pleased to know Aegislash can control even multiple people at once. Celosia reminded she did attempt to do the same in Santalune City, but Korrina's Mega Lucario emitted an aura to stop the mind control. Malva told Aegislash was needed to control the people to move the Xerneas tree to the headquarters. After coming back to the headquarters, Celosia immediately sent Aegislash to control all the prisoners, who nearly escaped the headquarters. Celosia came back to the Xerneas tree, with Aegislash controlling the prisoners. However, once X, Y, Tierno, Shauna and Trevor revealed themselves (with their ambush having failed), X sent his Elec and Kanga to attack. Essentia had Trevenant entangle the group, while Aegislash proceeded to attack the children with Slash. Just then, Gurkinn's Gengar appeared and stopped the attack, as Gurkinn, Korrina and Diantha came to stop the moving of Xerneas tree. Korrina battled Celosia and had Mega Lucario attack Aegislash, since she also wanted to protect the children of Vaniville Town. Korrina had Mega Lucario attack Aegislash with Bullet Punch, while Celosia was pleased to battle a good opponent. In the end, Mega Lucario and Aegislash exchanged blows and both sides were defeated. Celosia, before fainting, realized Mega Lucario used Bullet Punch, to reserve its power to emit the aura and stop people from being mind-controlled.