Mantax was one of the six leaders of the League of Six Kingdoms. He led many attacks with his army, all of them successful. He and the other members of the League, along with their armies, led an attack on the Great Spirit Mata Nui, but were stopped by the Brotherhood of Makuta and were about to be executed when Botar came and transported them to The Pit.
Mantax and the other Barraki spent a long time there, until the Great Cataclysm, which shattered the jail, allowing him and the other Barraki to escape. They then roamed around the pit separately, though still working together. Mantax formed an army of rays.
Their opportunity for revenge came when the Ignika floated down to Mahri Nui. Mantax did not know about this until later, when he was taken by Kalmah to try and stop Dekar, who had the Mask of Life. They discovered Dekar in a cave, and Kalmah went in close while Mantax waited behind. Then the Ignika created a huge Venom Eel to stop Dekar from destroying it, and it slammed in to Kalmah the force knocking him out. Mantax reported to Pridak what happened, and they both went there to investigate. The other Barraki were there too, as well as Brutaka, but he was taken away by a Giant Squid.
Mantax and the others went in and got the mask from Dekar, setting off a bright light that enveloped all of Mahri Nui. Later, when everything was settled, Mantax continued his daily work of looking for the Betrayal Contract that told which Barraki betrayed them to the Brotherhood of Makuta. However, Takadox and Kalmah found out and went to see more, but all three Barraki were found by a group of Zyglak. Luckily, Takadox tricked them and they all got away in time for the arrival of the Toa Mahri.
Once they took the Toa Mahri away, they put them in a confined cell, but they all got out. The Barraki came up with a way to solve this problem-to split up and find them. Mantax found Toa Mahri Hahli, and defeated her using stealth and his paralyzing head spikes. Later, Hahli escaped, and fought Mantax again, but this time managing to trick him into thinking that the other Barraki were allying and going against him. This caused Mantax to promote Hahli to his general.
Later, Mantax and Hahli went to stop Hydraxon from destroying the Ignika. What followed was a complex battle that left the mask in the claws of Mantax, giving him the power to take away energy from others close to him and give it all to him. He stopped Hahli and Hydraxon by using this power, and told them to get the other Barraki ready for a meeting. He then left and got ready, bringing the Ignika and the Betrayal Contract with him.
At the meeting, he told the Barraki about both the Betrayal Contract and the Ignika, and was about to give them a list of demands when an avalanche hit, causing Mantax to dive for cover, only to be stopped by Takadox in the chaos, who announced that he was the betrayer, and that Mantax was about to die. However, Jaller and Matoro arrived and stopped Takadox from killing Mantax. However, this gave Mantax a chance for revenge, and he jabbed his head spikes into the blue Barraki's abdomen, not killing him, but severly wounding him. But with Takadox out of the way, it was easier for the two Toa Mahri to knock him out. When Mantax, woke up, the Barraki had gathered together for a final attack on the Toa Mahri, but Teridax, disguised as a Maxilos, got in the way, easily defeating the six of them, but their armies had gathered by now, and destroyed Maxilos, though Teridax had managed to escape.
The six Barraki then headed for the Toa Mahri. They found them, and commanded their armies to attack with as much force as possible. However, the Toa Mahri were transported away to Metru Nui by the Ignika before the Barraki's armies could destroy them.
Mantax and the other Barraki then tried to avoid capture. Mantax found the original Hydraxon's helmet, and gave it to Pridak. He and Pridak were the last two Barraki to be captured.
He was later set free along with the other Barraki and helped with the Order of Mata Nui's war efforts against the brotherhood.
After the war was over, Mantax sheltered on Xia with the other Barraki and began to reform the League of Six Kingdoms.
After Teridax's death, he and his newly formed legions evacuated the Matoran Universe to Spherus Magna. His mutations were later undone by the Ignika.