Blackstone Fortress

The Blackstone Fortresses were originally created as weapons in the first war against the C'tan, and were known to the ancient Eldar as the Talismans of Vaul. To capitalise on the C'tan's vulnerability to warp attacks the Fortresses were equipped with a warp-cannon that could create a devastating rip in space and an eruption of energy out of the Matterium. Somehow six Fortresses out of however many were originally created were left abandoned and drifting in Imperial space in various systems through the Gothic Sector, and were given the name Blackstone Fortresses when humans discovered them and incorporated them into their system infrastructure.

The Fortresses that humanity discovered were dormant and were easily boarded and occupied: there was enough energy in their power grids that the humans were able to live in them and turn them into powerful spaceborne hardpoints that were central to the fortifications in the systems they occupied. However, during the Gothic War Abaddon managed to use a pair of ancient devices he had recovered to rouse the Fortresses and bring them under his control.

Once awake they were able to shed their grafted-on Imperial equipment and glide slowly through space under their own power, using their warp-cannon in place of the turrets and fighter bays the Navy had built onto them. There's also a record of a boarding party that made its way into one of the activated fortresses and described it as like a living thing, pulsing with power. As the war progressed and Abaddon acquired more fortresses he was able to combine their warp-cannon attacks to obliterate entire stars.

At the end of the Gothic War Abaddon retreated into the Eye of Terror with a small number of activated Blackstones (the exact number is unknown but is estimated at two or three) and all those remaining in Imperial custody simultaneously disintegrated. Abaddon seems to have been able to retain his long enough to bring them with him on the Thirteenth Black Crusade.