Acronix is a brash, confident warrior and a master of hand-to-hand combat. Having spent forty years outside of time, he is fascinated with all the innovations that have come about while he was trapped in the Time Vortex. 
The epilogue of The Dark Island Trilogy features Acronix alongside his brother Krux in a vision experienced by Sensei Wu, who describes the pair as "the twins I thought lost to time."
Having faced Garmadon and Wu forty years previously, Acronix emerges from a vortex above the Monastery of Spinjitzu only to be confronted by Wu, whom he is surprised to see looking older. The two soon face off, with Acronix initially confident but then forced to yield as Wu drives him onto a ledge with only Wu preventing him from falling. However, one of the Time Blades then emerges from the vortex as well, and Acronix seizes it in order to continue the battle, which quickly turns to his advantage. Having used the blade's power to travel forward in time several times to evade Wu's blows and strike back at him, he lands a devastating strike he refers to as the "Time Punch."
With Wu showing signs of weakness, Acronix attempts to finish him off with another blow but fails, but then leaves Wu stranded on a branch sticking out from the monastery mountain. Before he can depart, however, the six Ninja arrive in the Destiny's Bounty and engage him, though he outwits them while commenting that Kai isn't the first Master of Fire he's ever faced. After the Ninja succeed in coordinating their attacks against him, he creates the illusion of being destroyed, in reality making his way to safety. Acronix then travels to the Ninjago Museum of History, where he is at long last reunited with his brother Krux, who has been living under the assumed name of Dr. Saunders.