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Unggoy Heavy wear distinctive emerald armor with a full face methane breather. In Halo: Reach they have a cylindrical methane tank on their back (similar to the Unggoy Ultra) and are a darker shade of green. In Halo 3 they have a triangular methane tank and do not wear a full face mask. They can take more damage than Unggoy Minors and Majors, but not as much as higher-ranking Unggoy Ultras.
Unggoy Heavy are usually seen in conjunction with other Covenant infantry. They usually can be found operating as heavy support units and are usually operating heavy weapon systems such as Plasma Cannons, Shade Turrets, and Fuel Rod Guns with deadly effect. In Halo: Reach they are commonly seen operating the plasma cannons on Wraiths.
They will usually not flee from their position unless the player is too close or out of range of their weapons, in which case they will pull out a Plasma Pistol and attack like a normal Unggoy. They are easily dispatched using most tactics. Their armor seems to be slightly tougher than the average Unggoy, most likely to protect them from explosives and the possible backfiring of their powerful weapons. Unlike most Unggoy, it is rare for Unggoy Heavy to throw Plasma Grenades.
Note that the cannon is free for a player to use after taking out its operator. In Halo 2 a Unggoy Heavy must first deploy its Plasma Cannon before it can be used. If the Unggoy Heavy carrying the Plasma Cannon is killed before it can deploy it then it cannot be used. This is not the case in Halo 3, since most of the Plasma Cannons encountered are already deployed, and they can still be used even if they are detached from their tripod mount.
In Halo 3, Unggoy Heavy are seen utilizing many different heavy weapons. They are commonly seen driving Ghosts and seem to be skillful drivers, especially on higher difficulties. Due to their low target profile, they are also much harder to kill in a Ghost from head on. Unggoy will attempt to run over enemies and will keep their distance from advancing foes. Unggoy Heavy may also work in teams on higher difficulties in which case one rams a foe and another shoots the foe out of what they are driving. Usually, if they're not piloting a Ghost then they're manning a plasma turret or carrying a Fuel Rod Gun. Like Spec Ops and Ultra Unggoy, they are some of the most dangerous and tactical Unggoy. When not manning a vehicle or turret or carrying a Fuel Rod Gun, Unggoy Heavies will usually use Plasma Pistols or Needlers.
- Unggoy Heavy are sometimes, but rarely, seen wielding Spikers. This is easier to find when the player destroys a Phantom, the Unggoy will most likely (if it survived) be wielding a Spiker.
- Unggoy Heavy are classified as Spec Ops Unggoy in Firefight mode.
- Unggoy Heavy have been seen wielding detached turrets, even carrying them with one hand, possibly because they are stronger than normal Unggoy.
- In Halo: Reach, Unggoy Heavy have a cylindrical methane tank, like the Ultras, and a full face mask, like the Spec Ops, that looks similar to a Human gas mask.
- Some Unggoy Heavy in Halo: Reach have been seen using Plasma Rifles, notably on the mission ONI: Sword Base, and when they operate as a Wraith's gunner in Campaign and in Firefight.
- Unggoy Heavies can be seen coming out of Covenant Drop pods in Halo: Reach.
- Unggoy Heavies often seem to be wielding more powerful weapons when they are found arming a turret.