The Black Fortress of "Krull" is a mere childish nuisance compared to the threat posed by this spacecraft-turned-evil-stronghold.
Yes, behold the Incû-Holoinas, the Ark of the Skies. This golden vessel crash-landed in R. Scott Bakker's saga "The Second Apocalypse," inflicting a vile and hedonistic race known as the Inchoroi on an unsuspecting fantasy world.
Though partially buried in its own impact crater, the ship survived its descent to the surface. The ark's twin "horns" became the towers of Golgotterath, which serves as a darker vision of Tolkien's Mordor. From its haunted depths, the Inchoroi launch everything from sinister deceptions to open wars of extermination against the planet's human population. More than 4,000 years of horror ensue.
What do the Inchoroi want, other than to have a good time and spread suffering? Oh, they just aim to cleanse the planet of souls, seal the world and hide their sins from divine judgment. That sort of thing. The interior of the Incû-Holoinas is best described as a maze of obscenities, where the remaining crew work alongside twisted and once-human sorcerers to spawn monstrous armies.
All attempts to destroy the Incû-Holoinas have failed.