Fleetmaster

The Fleet Master is a high-ranking Elite within the Covenant. This grade of Elite commands Task Forces and small fleets.

Rank
Fleet Master is the highest office for an Elite except for Supreme Commander, or perhaps an Imperial Admiral. They are one step higher than an Elite Ship Master. Whenever a small fleet is needed for any mission away from the entirety of the Covenant fleet, a Fleet Master will be assigned and given command of several starships during that task. This shows they could be the Covenant equivalent to a UNSC Rear Admiral. Since Fleet Masters are such a high rank, they may also command the entire fleet should the Supreme Commander or the Imperial Admiral be incapacitated during space combat, though this is still fully unconfirmed.

They normally operate from a large warship, as a Reverence-class Cruiser. This can especially be seen in Halo: Ghosts of Onyx, as the Imperial Admiral Xytan 'Jar Wattinree commanded a Supercarrier, while the Fleet Master (Voro Nar 'Mantakree), operated from a Reverence-class Cruiser.

The Arbiter, should their role ever require it, could probably be assigned the role of Fleet Master. As well, should Councilors decide to get involved in military operations, they too could probably have the authority of rank to command a fleet, even having Zealots subordinate to them.

Only one Fleet Master is officially known, Voro Nar 'Mantakree, who commanded a small task force to the planet of Onyx in order to accquire the holy relics and technologies from the planet. Halo: Ghosts of Onyx, Page 243

Physical Appearance
Fleet Masters don't appear to have any changes on their armor from other Elites of the same rank (it is impossible to tell a Fleet Master apart from a Zealot based on armor alone).

Trivia

 * When Voro 'Mantakree was promoted from Major Domo to a Zealot Fleet Master, an extra part was added to his name, making it Voro Nar 'Mantakree.
 * A Fleet Master may potentially have more authority than a Field Master, since delivering an army to the battlefield may fall into one of the responsibilities performed by a Fleet Master.

Known Fleet Masters

 * Voro Nar 'Mantakree - Ship Master of the Incorruptible, who led a fleet to follow the Bloodied Spirit.
 * Rtas 'Vadum - Ship Master of the Assault Carrier Shadow of Intent.