In the Halo series, what fleet was Thel 'Vadam supreme commander of before being branded an Arbiter?

Thel 'Vadam, who later became known as the Arbiter, was the Supreme Commander of the Fleet of Particular Justice in the Halo series before being branded as such. This fleet was a powerful and influential Covenant fleet, responsible for carrying out military campaigns and enforcing the will of the Prophets. However, after engaging in an unsuccessful battle against humanity on the planet Reach and failing to secure the Halo ring, Thel 'Vadam was deemed unfit by the Prophets and was branded as the Arbiter, a title that carries the duty of carrying out the most dangerous and suicidal missions on behalf of the Covenant.