Essentia used Trevenant to surround the Tower of Mastery with its roots, as well as attack Grukinn using its roots. Korrina faced Trevenant and had Mega Lucario battle it. Trevenant used Bulldoze, which wounded her Mega Lucario and even shattered a part of the stairs. Mega Lucario grabbed Trevenant's roots to prevent it from repeating Bulldoze, but got burned by Trevenant's Will-O'-Wisp attack. Korrina soon realized Trevenant, being a Ghost-type Pokémon, was immune to Mega Lucario's attacks. Trevenant, however, continued extending its roots, to grasp at the Mega Lucario statue, inside the tower, by absorbing the nutrients from the ground. Mega Lucario eventually freed itself and jumped down, which nearly hit Essentia. Korrina thought she was mistaken, seeing Trevenant was actually controlling the plants in the area to attack them. Korrina and Gurkinn triggered the self-destruct mechanism of the Mega Lucario statue, so Essentia fled with her Trevenant away.
X, Y, Tierno, Shauna and Trevor tried ambushing Celosia and Essentia, who tried to move the Xerneas tree. However, Essentia had her Trevenant control the plants and entangle the children, allowing Celosia's Aegislash to attack X. However, the attack was blocked by a Gengar, for Gurkinn, Diantha and Korrina arrived to help the children out. X managed to get his new Gengar, Garma, to attack Trevenant with Shadow Punch. Just as Gengar attempted to attack once more, Trevenant dodged, since it can predict attacks. However, X Mega Evolved his Gengar, who came out of Trevenant's shadow and hit it with Shadow Punch. This made Trevenant faint, as well as to free X's friends. Soon, Gurkinn explained to X Gengar's Levitate ability was swapped with Shadow Tag upon Mega Evolving, which made Trevenant unable to be called back.