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The purpose of the Citadel is unknown. Like most Vex structures, it is made of enormous rectangular stone blocks interwoven with machinery; according to some, the stone used in its construction is eons old, yet it contains technology vastly surpassing humanity's own. At the top of the Citadel lies The Terminus, a gate network that can transport the Vex throughout all other time streams connected to the Vault of Glass.
During the Wolf Uprising, Skolas, Kell of Kells led the remnants of the House of Wolves in a final stand atop the Citadel, where he attempted to use the Vex gate network to pull the entirety of the House of Wolves through time to the present. He was ultimately defeated by the Guardians.
Later the Taken forces lead by an Echo of Oryx began to roost in the spire, only to be driven out by the Guardian.
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