The Blackstone Fortresses are six ancient space-based super weapons said to have been forged by the Eldar smith god Vaul to destroy the Necron star gods, the C'tan. For this reason, they are sometimes called (especially by Eldar) the Talismans of Vaul. The C'tan were basically all-powerful in the material realm, but they were extremely vulnerable to the warp, so the Blackstone Fortresses were given a massive warp cannon as their primary weapon. During the Gothic War one Fortress scoured a planet; three of them combining their beams destabilized a star, causing it to go supernova a month later and destroy the system.
It seems Vaul had a particular rival in the Void Dragon, a powerful C'Tan who possessed control over material machinery. With these weapons, Vaul went to confront the Dragon and although the outcome is unrecorded, the Dragon still lives (although sealed away) so we can guess Vaul suffered epic failure. It is also possible that the combined fire of these fortresses made the Void Dragon weak enough to retreat to ancient earth and start eating virgins, though even these things could not kill it. To add insult to injury, taking codices literally would mean that rather than the Void Dragon himself, it was just a shard of him that was blasted away.
After the Fall of the Eldar, the Fortresses were left adrift in the Gothic sector, where they were found by the Imperial Navy. The Navy moved in and turned them into naval bastions for Battlefleet Gothic, despite being unable to activate most of its armament. However, Abaddon found some artifacts that allowed him to assume direct control of them and managed to swipe three of them, kicking off the twelfth Black Crusade, before the Navy and the Eldar stopped him. The Imperial and Eldar navies then chased after the fortresses and managed to take one back, but then Abaddon caused it and the three still in Imperial control to self-destruct.
The Blackstone Fortresses were mentioned during the 13th Black Crusade, one was damaged and retreated during the space battle over Cadia (It annihilated an Imperial Navy battlegroup as well as Eldar and Necron fleets and only retreated after reinforcements from Segmentum Solar rushed in) while the other was rumored to be destroyed in transit by the Necron fleet. However, this was retconned and now the current fate of the Blackstone Fortresses is unknown. The two unaccounted-for fortresses were not present when the other four self-destructed, so they might be lurking around still...
Fall of Cadia confirms one of them was still in Abby's control. Sick and tired of Creed's awesome bullshit and panicking after he sees an Archmagos activate the Necron anti-warp pylons that threaten to close the Cadian Gate forever, he rams the planet with his Fortress as a last ditch gambit to blow the pylons and kill the last Cadian garrison. The Cadians are barely scratched.
Rise of the Primarch reveals the whereabouts of the last Blackstone Fortress. Apparently Abaddon had gifted it to Huron Blackheart.