The Sky Fortress (or "Skye") was a massive fortified gunship belonging to the Imperial Fists at the time of the Horus Heresy. Little is known about the Sky Fortress despite its important role in the history of the Imperium.
During the Horus Heresy
The Sky Fortress was once an orbital plate (known as Skye) that was the sole large platform to remain above Terra in the lead up to the Battle of Terra. Cannons were added to its frame, and it was kept in geosynchronous orbit to the West of the palace.
During the peak of the Siege of the Imperial Palace, the Imperial Fists and Rogal Dorn used the Sky Fortress to make a massive move to reinforce the Palace defenses after the traitor forces broke through the Palace walls. After facilitating the redeployment of Dorn and his Imperial Fists, traitor Titans targeted the great ship, destroying it. The crew of the Sky Fortress piloted its final descent into Horus's forces below. The impact detonated the ship's plasma reactors, creating a crater three kilometers across, inflicting heavy fatalities and searing eyes with the flash of its explosion.