From Halopedia, the Halo wiki
An Excavator is a class of walker constructs established by the Covenant and later used by its remnants. As their name implies, vehicles of this class were used as excavators and mining platforms. Excavators were also used by the Covenant as heavy weapon platforms, mobile bases, and combat engineering vehicles.
The Excavator class encompasses several lines of vehicles, including the Harvesters, Locusts, and the comparatively ubiquitous family of Scarabs. Excavators often make extensive use of Lekgolo, with the worm colonies bound together with the vehicle's control systems. The Lekgolo were rarely given complete control of an Excavator, and a supervising Covenant officer and military personnel usually accompany the vehicle. The Lekgolo meta-colonies that operate Harvesters from within are known as the Sbaolekgolo.
All Excavators were organized by the Covenant into several tiers, each one indicating a walker's size, excavating efficacy, and delicacy. Nine tiers of Excavators are noted in records that survived the Covenant Empire's collapse in 2552, though humanity has only encountered three as of 2558.
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