Mesak is a Virnahz created by Baxitus, from Mesograk's homeland, Oohrinah. He was the supposed "son" of Mesograk and served the Confederacy during Mesograk's rule. Before Mesograk took over the Confederacy, Mesak actually knew Onatu fairly well (in which coincidencially, he was the "son" of Zokorak). He also knew another member (who was a member of the Virnahz species) named Advlukk.
Mesak was very loyal to his leader, Mesograk. He proved to be so loyal that he was entitled as "The Son of Mesograk", proving how much he was willing to do virtually anything that his leader would command from him. He is a high-ranking commander, who has a second-in-command, who is a strange unknown entity, even to Mesak; known as Megdeth. He and Megdeth currently lead an elite team of COI memebers, along with zombie-like creatures that Mesak customly created known as Zokead's (a related zombie-like creature to that of Mesograk's Zombrak's/Skeletak's).
He'd one day confront a Toa named Kylord and battle him. During the battle, Kylord briefly used his mask power that somehow defeated Mesak. Mesak was greatly interested in Kylord's mask power and also saw the power of his mask to somehow outstand the power even of his "father", Mesograk's Mask of Immortality.
During the war between the Order of Mata Nui and Brotherhood of Makuta, Mesak ordered Megdeth to save "Brutal"'s Toa Team from the Brotherhood, as he wanted all of the Toa alive (as he wanted the Toa to be able to defeat the Makuta in Karda Nui, seeing them as the more bigger threat to him). Once he did that, Mesak confronted "Brutal", Cyrax, and Airko and ordered them not to interfere in his attempt to capture Kylord. Unfortunately, "Brutal" and the others were able to regroup with Kylord before Mesak could confront him. Mesak took dramatic measurements, and commanded Megdeth to capture one of the Toa's with the exception of Kylord; Megdeth did just that and chose Cyrax. After Mesak easily defeated Kylord's allies, Kylord was being force to surrender his Kanohi mask to Mesak, but before he did that, Mesak also decided that he wanted his soul to be crushed. So he ordered Megdeth to kill Cyrax, so he stabbed her through the back, seemingly killing her.
After the "death" of Cyrax, Kylord was in rage and started to unleash it all on Mesak. Mesak would fight back, however, and the two would duel until Mesak turned out to be the victor of the fight. However, Cyrax appears (thanks to her "special" Faxon, keeping her alive) and grabs Mesak's jaw and rips it off of his face. In pain, Mesak could not continue his mission, as he was to wounded to do so. Megdeth teleported himself and Mesak out of Karda Nui and back to their base.
Thank's to Mesak's power to regenerate, he was able to regenerate a new jaw for himself. He also decided to upgrade his armor to be more tougher for any foes, such as the Toa, to take him down. He is currently at his base, along with Megdeth and his group of COI members.