To face me is to face thy deepest, darkest nightmares!
Diabolos is an antagonist from Final Fantasy XIV, first appearing as the final boss in the level 50 dungeon, Lost City of Amdapor, and later in Final Fantasy XIV: Heavensward as a major antagonist in the "Shadow of Mhach" storyline.
Diabolos is a large, winged, demon-like entity, primarily red and black in color.
Diabolos rarely speaks, and when he does it is usually to offer threats and intimidation to his enemies. He seems to be a leader among Voidsent, as other powerful Voidsent, such as Ferdiad, refer to him as "Lord Diabolos" and follow his command.
Diabolos was a powerful Voidsent, summoned to Eorzea by the Voidmages of Mhach during the Fifth Astral Era. In the waning days of the era, during the destructive War of the Magi, he was sent to attack the city of Amdapor. He destroyed the city, but the White Mages of Amdapor magically sealed him in the ruins. Thus confined, he slept for the ensuing centuries.
As the magics that sealed Diabolos began to weaken, powerful dark forces could be felt emanating from the ruins of Amdapor. E-Sumi-Yan and Raya-O-Senna send the adventurer to the Lost City of Amdapor to investigate. Upon reaching the final chamber of the city, Diabolos' seal breaks. The Voidsent seems disoriented, confusing the adventurer's party for the White Mages that sealed him, before attacking. Upon defeat, he states that his long sleep has weakened him and he is not yet at his full power before disappearing, seemingly vanquished.
Some time later, the adventurer teams up with a a Sky Pirate named Leofard and his band of Redbills to investigate a mysterious airship, dubbed the Void Ark, that has drifted through the skies for ages. On board, they are confronted by a multitude of Voidsent that infest the vessel. They meet Diabolos, who unleashes even more Voidsent to attack the adventurer and their companions.
Though they defeat Diabolos' servants, they cannot prevent him from taking a large, coffin-like artifact, and using the Ark's power to create a void gate through which he escapes with it. The adventurer and the Redbills learn of the Ark's history from the magical creature that had been guarding it, Cait Sith. They are told that the coffin-like object Diabolos took contained a powerful Voidsent known as Scathach, the Shadow Queen, who Diabolos likely intends to revive.
When the adventurer and the Redbills journey to The Weeping City of Mhach in search of a powerful artifact that can be used against Scathach, Diabolos sends Ferdiad to retrieve the artifact for themselves. After Ferdiad fails, he reports to Diabolos, who departs to finish the preparations for his coming plan.