Born Thel 'Vadam, he became Thel 'Vadamee upon joining the Covenant military. He was the Supreme Commander of the Fleet of Particular Justice, which laid waste to the human colony planet of Reach, and which followed the Pillar of Autumn as she made her escape. Thus he, along with the humans, discovered the Halo artifact. Unfortunately, he could not prevent its destruction at the hands of the Master Chief.
As a result, he was stripped of title and rank, and nearly sentenced to death. He was saved by an appointment to the position of Arbiter, tantamount to an honorable death sentence. He initially did the work of the Prophets, but after learning the truth about their "Great Journey," he defected, and the Covenant Civil War began. He eventually fights alongside his old enemy, the Master Chief.