"It is the very embodiment of a predator; a soulless, remorseless beast which carries with it the weight of uncounted alien minds all hungry for our flesh."
The Dagon Overlord is a powerful Tyranid synapse creature similar to a standard Hive Tyrant that has been seen leading the Tyranid assaults of Hive Fleet Dagon in the Jericho Reach. It is unknown whether the Overlord is a new type of Tyranid bioform that will be deployed by the Hive Mind in all Hive Fleets, or whether it is a single creature wholly unique to the activities and circumstances of Hive Fleet Dagon.
The first sightings of the Tyranid Dagon Overlord, or simply the Overlord as it became known, came in conjunction with the first victories scored by Hive Fleet Dagon in the Jericho Reach, as if the Hive Mind had taken the measure of the foes it has faced and chose to evolve a specific tool for their eradication. Other theories speculate that the Overlord was a genus specific to that Hive Fleet that existed long before its arrival in Imperial space, and that it was simply a superior warrior construct, brought out of storage to face a more stubborn than expected adversary.
Whatever the case, the Overlord first appeared during the opening days of the battle for Castobel, leading a mighty swarm against the amethyst spires of the Trimalov Hive, rending through its walls and dispatching its defenders in a month-long orgy of carnage and slaughter. Those fortunate enough to escape the fall of the hive city with their lives took with them tales of terror and the memory of a beast which stood apart from even the worst horrors the swarm has to offer.
The Dagon Overlord is in appearance similar to a Hive Tyrant, but is different enough to stand apart from its lesser brethren and be remembered by those that see it looming across the battlefield. Covered in slimy plates of chitin which constantly drip and leak fluid, the Overlord projects an aura of horror and menace like nothing else within the swarm. Its importance and power are also readily apparent when it strides alongside other Tyranid creatures, its powerful presence emboldening and maddening the lesser creatures more so than would be expected by any link to the Hive Mind. It is almost as if the Overlord is the embodiment of Hive Fleet Dagon's ire toward the Jericho Reach and its alien hatred infects creatures with a personal desire to murder, maim, and kill all those in their path.
Since its first appearance, the Dagon Overlord has been sighted in almost all of the major combat zones of the Orpheus Salient, leading swarm after swarm into the ranks of Imperial defenders. This has led many to believe that the Overlord is not a unique creature at all, but instead represents a new bioform of Tyranid. However, whether it is a unique entity or not is hard to determine when dealing with the Hive Mind, as it is constantly recycling its troops. This means the Overlord may well be a single veteran of many wars, and may have even fallen countless times to the foe, only to be have been re-spawned by the Hive Mind.
Both the Ordo Xenos and the Deathwatch have made attempts to find the truth about the Overlord's existence. Neither has met with much success, and both Inquisitorial agents and Battle-Brothers have been lost during the process. Even when the Dagon Overlord seems defeated, it invariably rises again in another place at another time.
- Deathwatch: Mark of the Xenos (RPG), pp. 33-35