The Scarab is different from most of the Covenant's other ground-based vehicles. Instead of using anti-gravity pods, it walks about on massive legs. It is highly mobile for a vehicle of its size, and sports two turrets that shoot large red plasma similar to the Shade, as well as a large, forward-firing plasma cannon. This cannon is also somewhat reminiscent of the Gravity Cannon that the hunter's carry.
The gun is extremely powerful, and is not a practical application for battle, except in battles that involve tight spaces such as cities. Rather, the Scarab is used to melt away miles of rock in order to excavate, no doubt for the structures and artifacts of the Forerunners.
While previously thought of as a mere mining digger, the Scarab's purpose as an assault vehicle has been expanded hugely; at least two Scarabs are seen in Halo 2, while they also appear in the Halo Graphic Novel and will be in Halo Wars. They appear to be major Covenant foreward assault platforms, causing mass destruction and chaos in preparation for more ground troops to fight. They also seem to be capable of excavation as well; their main cannon is extremely powerful.
The Scarab is somewhat similar to Mörser Karl mortar or railway guns.
Appearances
During The Battle of Reach, Spartan-104 describes seeing several large "insect like" excavating machines digging up CASTLE Base. He destroys them using Plasma Mortar from a Wraith tank.
When the Covenant first travel to Earth, led by the Prophet of Regret, they believe the Human homeworld was simply an abandoned Forerunner world, and so they do not expect to meet Humanity on it's own homeworld. A Scarab was seen in Old Mombassa, with the intent of searching for an artifact. Caught off guard and forced to fight on the human's terms, the Scarab is repurposed to crush any resistance in its path. It is eventually cornered and destroyed by a group of Marines with the help of the Master Chief. However, the caption of Turf suggests that there may have been several Scarabs present at the battle, and the wreckage of a second Scarab is seen in Mombasa on that level.
Another Scarab was on Installation 05 near the Control Room. It was commandeered by Sergeant Johnson, along with two Marines, and used to break into the Control Room of the Halo.
Scarabs will also apparently make an appearance in Halo Wars.
Scarab Gun: Secret Weapon in Halo 2
The Scarab Gun is a weapon that players can pick up; it can be found on several of the levels where the scarab is encountered.
Weaponry
(2) Mounted Anti-Aircraft Turrets
Ammo: Super-Heated Plasma, (possibly have needler homing technology) Rounds Per Minute: 400-600 rounds/minute Notes: Presumed Computer Controlled, but a close look indicates that it may be manually controlled additionally.
(1) Front-Mounted Cannon
Ammo: Super-Heated Plasma, presumably un-guided. Rounds Per Minute: 10 second stream; 30 to 40 per second Max-Temperature: ? Movement of cannon: Through careful examination, one can see that the inside gun part seems to move forward from a cradle as the flaps open up. Notes: The projectile emmited from the Scarab's main gun emits a neon-green/bluish glow, indicating extremely high temperature. It's force is enough to destroy any combat vehicle in a fraction of a second, and could presumably melt through an interstellar war-ship's armor with the ease of a Covenant's capital ship's plasma bomb.
It can only be found on two levels.
Physiology
Height (standing, toe to highest point.): ? Width (standing, front to rear toe): ? Length: 64.9 Meters Weight (unloaded): ? Speed (unloaded): ? Materials: ? Shielding: No energy sheilding is known of. Armour Alloys non-identified.
Other
Deployment: Orbital Drop. Crew: At least 5 Elites, a white-armored commander among them. Grunts and Jackal are seen in greater numbers above and below deck.