The Black Throne is a massive spire located north of The Ashlands. It is bound to the demon commander Straga, it is also home to the three Tower Guardians (Bosses that test your understanding of the Voidwalker.), and the prison of Azrael, the Angel of Death. This is what War sees as his ultimate destination until he finds more information about what is happening from Azrael. More and more information is given upon the acquisition of each beam, revealing more and more about what exactly caused the events of the Apocalypse. When Straga is destroyed, the gigantic Spire apparently breaks apart, because, as Azrael informs War, Straga is a load-bearing boss. However, the top of the Spire remains in the air, possibly due to The Destroyer's dark influence. This is where the final battle against The Destroyer takes place.
War receives the Voidwalker in this dungeon, and it plays an essential part in redirecting the beams that are used to free Azrael.