The Blackstone Fortresses were a collection of six massive and mysterious artefacts of pre-Imperial culture that were scattered across the Gothic Sector in the Segmentum Obscurus. Each was a vast installation whose mass was larger than even the mightiest Imperial starforts and segmentum fortresses. Imperial archaeo-historians place the construction of these ancient artefacts anywhere between 17,000 and 300,000 standard years before their discovery in the 33rd Millennium. Other Imperial scholars disagree, estimating them to be far older, dating back to the early years of sentient life in the galaxy many millions of Terran years ago. For as long as the Imperium has been aware of their existence, the Blackstone Fortresses lay in a dormant state, with all but passive power systems shut down de