Thel 'Vadam, aka Thel 'Vadamee, aka the Arbiter, is a Sangheili leader, rising in the ranks under the Covenant, before becoming the Arbiter and allying with the UNSC late in the war. 'Vadamee was relatively low in the Covenant ranks during the Battle of the Rubble in 2535, but rose to commander of the Fleet of Particular Justice soon after. His ability to evolve his tactics to defeat the enemy made him a serious threat to the UNSC, leading to an ONI plan to assassinate 'Vadamee. 'Vadamee was a fleet commander in the Fall of Reach, and led the same fleet at the Battle of Installation 04. The loss of the Halo led to the hierarchs chastising 'Vadamee and assigning him to the role of Arbiter. It was as Arbiter that he learned the truth of the great journey, and became an ally of the UNSC during the Battle of Installation 05. With the help of the Master Chief, 'Vadam assassinated Prophet of Truth Ord Casto, ending the Human-Covenant War.
After the war, 'Vadam became leader of the Swords of Sanghelios, a group of Sangheili vying for control over Sanghelios. Though 'Vadam fought for many years for control of the planet and all Sangheili, he eventually put an end to the last significant threat to his rule in Jul 'Mdama's Covenant at the Battle of Sunaion in 2558.