Acronix is the main antagonist along with his twin brother, Krux, in Ninjago: The Hands of Time.
Long ago Acronix and Krux were the Elemental Masters of Time in the time of Serpentine War. When the war ended Acronix and Krux betray the other Elemental Masters, and then the other Elemental Masters tried to stop them.
Then Wu had an idea, he asked his friends Ray and Maya to make two Time Blades which had to be taken the powers of Acronix and Krux. The Time Blades brought their elemental powers and Wu and Garmadon make the time vortex where blades were destroyed. Then Acronix and Krux attacked the rear of the blades and they were thought to have disappeared into the vortex, later it became clear that before Acronix disappeared was a visible sign that told when he would return, it was forty years. And when Acronix returned he and Wu began to fight, and then Acronix hit him with the Time Blade.
When he met with his Brother they began to snatch the builders of Ninjago because they needed them to build the armors, the weapons and the vehicles from Vermillion Army. And they needed all four Time Blades so they could control all the time. When Kai, Nya and Wu met them on Iron Doom, Wu untie the return blade and gave it to Kai and Nya because Wu wanted them to help Ray. When Kai and Nya returned to the present day while Wu remained on the Iron Doom to defeat Acronix and Krux.
Acronix and his brother continued to battle their enemy before they were presumably defeated by him. It can be assumed he and his brother are still lost while Wu managed to escape the vortex though he was de-aged into an infant.