Arbiter

Thel Vadam
The Arbiter is the awesome leader of the Sangheili forces. He was once the Supreme Commander of a Covenant Fleet but was disgraced by the Prophet of Truth and made Arbiter. He vowed his vengeance and the Elites rallied by his side when the Prophet betrayed them. He killed Truth by slowly gouging him to death, and bore witness to Truth's last laugh against Humanity, which consisted of his head spinning off his body like a helicopter and biting Lord Hood's nose off. Arby was not even slightly fazed by this, nor was he fazed by the Gravemind's inevitable betrayal. Arby helped Johnny stop Guilty Guilty from destroying the Universe and was with him when they escaped the new Halo. However, Johnny was seperated from Arby and landed on Requiem. Arby now lives on Sangheilios with his pal, the Shitmaster, and his nine thousand million wives.

Other Arbiters
There was also another Arbiter that discovered a Shield World similar to Requiem, which was infested with Flood. He fought Captain Cutter but was eventually killed by Sergeant 'Awesome' Forge. His name was Ripa Moramee, and he was apparently eight feet tall, which is pretty tall.

According to the withered Prophet of Mercy, there were Arbiters that stopped a Grunt Rebellion and tamed a bunch of Hunters.

Youtube Clip
http://www.youtube.com/watch?v=Ii38-0hkZJQ