The assault on Vader's Castle, also known as the Mustafar raid according to the Imperial Security Bureau, transpired in 3 ABY. During this time, the Knights of Ren executed a daring heist under the direction of Lady Qi'ra from the Crimson Dawn syndicate. Their mission was to extract the Screaming Key from Fortress Vader, located on the planet of Mustafar.
After enlisting the Knights of Ren on Varnak, Lady Qi'ra tasked them with the secret mission of penetrating Darth Vader's vault within his fortress. Their objective was to secure the Screaming Key for her, acting on behalf of the Crimson Dawn criminal organization. Qi'ra furnished the knights with the necessary access codes and a detailed map of the fortress, which instilled confidence in their leader, Ren, despite the absence of a comprehensive plan.
The Knights of Ren boarded their starship, the Night Buzzard, and swiftly made their way to the Gahenn Plains on the planet of Mustafar. Riding their speeder bikes, the knights approached the fortress from its southern flank. Ren opted to utilize an ancient entrance to the Sith cave, which was surprisingly unguarded, as a means to infiltrate Vader's castle unnoticed. Although the entrance was obscured by flowing lava, the knights skillfully employed the Force to bypass the molten barrier and gain entry to the cave.
Upon entering the cave, Marinda, one of the knights, immediately sensed the location's potent connection to the dark side of the Force. Subsequently, Bazzra spotted what appeared to be Darth Momin's helmet resting on the ground. The mask, seemingly still containing the Sith Lord's severed head, activated before Bazzra, prompting her to obliterate the artifact with a blast.
After infiltrating the castle, the Knights proceeded past numerous bacta tanks until they encountered Vaneé, the attendant. Vaneé promptly summoned several lava troopers when the knights launched an attack. The knights effectively neutralized and contained the troopers, while Ren skillfully employed his lightsaber to forge a path and locate Vader's vault. Upon gaining access to the vault, Ren utilized the Force to discern the key's distinct screams and pinpoint its location among the numerous hidden artifacts.
The Knights soon observed that the majority of the lava troopers had departed, and they witnessed the arrival of the Dark Lord of the Sith, Darth Vader, at his castle. Vader, expressing his displeasure at the knights' uninvited presence, swiftly dispatched Fyodor with a Force-based attack. Ren instructed the remaining knights to retreat while he engaged the Sith Lord in a duel. During their clash, Vader questioned Ren's worth and suggested that he could still descend further into darkness. This prompted Ren to leap from the castle, utilizing the Force to control his descent and ensure a safe landing on the ground.
Following their successful escape from the castle on their speeder bikes, Marinda informed Ren that they had likely ascended on Darth Sidious's list of targets. Simultaneously, Vader remained amidst the wreckage of the battle within his fortress, contemplating the existence of the Knights of Ren. Coupled with other incidents, the Knights' successful breach of Fortress Vader led Sidious to conclude that the Sith had encountered a new adversary. Vader and Sidious subsequently discovered that Crimson Dawn was, in fact, orchestrating events against them. Meanwhile, the Knights, accompanied by "the Archivist," journeyed to the dark side hellscape with the Screaming Key, acting on Qi'ra's orders to retrieve the Fermata Cage.