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.
In The Hatching, Wu reveals more of Acronix and Krux's backstory, indicating that they were Elemental Masters who betrayed their comrades and sought to rule over Ninjago until their powers were taken from them by the four Time Blades, which were then thrown into a Time Vortex. The twins pursued the weapons and were thought to be lost, though Wu learned when Acronix would make his reappearance. Meanwhile, Acronix-who has fallen in love with the technological advances made during his forty years absence despite his brother's distaste for them-and Krux unleash their first Vermillion warriors, employing a plan that Krux has had in the works for the last forty years. After releasing one warrior-whom Acronix quickly realizes his brother has ordered to destroy various high tech devices in addition to kidnapping Cyrus Borg-they unleash several others to combat the Ninja upon their arrival. Krux then poses as Saunders and intercepts Zane as he is fleeing with Borg, allowing Acronix to swoop in and strike the Titanium Ninja down.
In A Time of Traitors, Acronix is introduced to the Vermillion overseers Machia, Commander Blunck, and Commander Raggmunk, as he and his brother continue to breed additional Vermillion for their plans. When Kai arrives at the museum looking for information, Acronix is eager to eliminate him but is prevented by Krux, who dons his Saunders disguise and tries to get him to leave. When this fails, Acronix attempts to go after him again only to be blocked by a Serpentine sarcophagus as Krux-his identity exposed-battles Kai; the Time Twins then end up in a battle with Kai and the arriving Nya. The ninja prove victorious, but Acronix is able to escape with his brother and Borg in tow.
In Scavengers Acronix and his brother dispatch the Vermillion to steal metal and kidnap various artisans from around Ninjago to advance their plans. In A Line in the Sand they redirect their forces after detecting the return of the next Time Blade to Ninjago, only for it to be captured by the Ninja. In The Attack Acronix and Krux reprimand Blunck and Raggmunk for using the Time Blade without authorization, but soon turn their attention towards reclaiming it by attacking the Temple of Airjitzu. They eventually engage the Ninja in battle directly only to lose their Fast Forward Time Blade, but then receive it and the Slow Down blade from Machia. As they make their departure, they take with them the ailing Wu, who is aging more and more rapidly due to the Time Punch's effects.
In Secrets Discovered, Acronix and Krux mock the captured Wu as they make their way to their swamp hideout, while also promoting Machia over the less effective Blunck and Raggmunk. In Pause and Effect they are able to intercept the Pause Time Blade thanks to Acronix's possession of a Borg Watch, one of his many attempts to convince Krux of technology's value. They then are able to ambush Kai and Nya after the pair locate Ray and Maya, who have been blackmailed into helping Krux in his plans for a number of years. In Out of the Fire and Into the Boiling Sea, the Time Twins travel with their captives and Machia to the Boiling Sea and force Kai and Nya to retrieve the final Time Blade so that it can be used to power the Iron Doom. Successful, they leave the four captured heroes and Wu to plummet into the sea and then board the Iron Doom with their forces before traveling into the past.
In Lost in Time, the Time Twins anticipate their arrival in Ninjago's past, unaware that Kai and Nya have snuck aboard the Iron Doom with Wu in tow. Emerging from the vortex, the villains face the Elemental Masters of the past and unleash their Vermillion forces, whom the younger Wu and his allies have no idea how to defeat until Kai and Nya enter the fray posing as their parents. Despite this, the Time Twins are able to achieve victory after taking control of the Vermillion directly and having them inhabit the Iron Doom to make it move, but depart for the future at Acronix's insistence after becoming annoyed with Kai and Nya's continued interference. They are once again followed by the siblings and a restored Wu, who rips the Reversal Time Blade out of the Iron Doom and gives it to Kai and Nya. He then pushes them out, returning them to Ninjago's present so that the blade can be used to heal Ray, as he and the Time Twins are lost in time.