From Halopedia, the Halo wiki
The Phelent-pattern is a heavy modification of the traditional Rizvum-pattern Revenant employed as a light tank by the Covenant. Unlike the Rizvum, the Phelent incorporates a light plasma cannon rather than the Rizvum's plasma mortar, and this reduced weight grants the vehicle a speed boost, allowing it to cross short spans of terrain extremely quickly. The vehicle's design differs in many areas compared to the original Revenant, with the hull loosely modelled on older-era templates so that it resembles the skull of the cara'kada blood-lizard.
The Phelent-pattern Revenant was originally created during the development of Halo 4. The vehicle was concepted by concept artist Glenn Israel and modelled by 3D artist Can Tuncer and was present in several early alpha builds before being ultimately cut from the game.
The Revenant has seen a major redesign since its iteration in Reach, with the vehicle as a whole being generally bulkier and up-armoured compared to the original version. Instead of the driver controlling a light plasma mortar, the vehicle now has an automatic plasma turret in the back which can be manned by an independent gunner - making the vehicle more akin to the Spectre or Warthog.
With the release of Halo 4 on PC as part of the Master Chief Collection, the Revenant was added to the game's files and can be spawned via modding.
Halo 4 in-game