Blackstone Fortress
The Blackstone Fortresses were ancient super weapons forged by the Eldar smith god Vaul to be a weapon to destroy the Necron star gods, the C'tan. It seems Vaul had a particular rival in the Void Dragon, a powerful C'Tan who possessed control over material machinery. With this 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.
In recent times Abaddon has moved into the Blackstone Fortresses and renovated them as his new holiday homes, complete with kitting them out with a lot of fancy new furnishings, such as fleshy corridors, bulging eyes and every other delightful chaos mutation. This was actually the work of that crafty bugger the Deceiver, who intended to place the Fortresses out of the reach of the Eldar for eternity, so they couldn't be used against him. Exactly why the Eldar left them just floating around in the first place then hiding them is a good question... Something about the Fall.
The current fate of the Blackstone Fortresses is unknown. Some have been destroyed and others are missing, so there might be at least one (possibly two) lurking around still...