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Vas'talos, previously known as Prefectia under the Imperium, is a Sept of the Tau Empire. While it was still ruled by the Imperium, Prefectia was a Fortress World known as the gateway to the strategically important Dovar System. During the Prefectia Campaign, the planet was conquered by the Tau under Shadowsun and declared a new Sept. The Tau have characterized the capture of Prefectia to be their greatest victory yet, but in truth the Imperium only regards it as a minor loss.