The Endymion Cluster is a star cluster composed of 20 star systems (6 of which contain worlds of major importance to the Imperium) located in the Maelstrom Zone of the Segmentum Ultima directly to the galactic east of Terra and very near to the Maelstrom Warp Rift itself. The cluster's worlds and peoples were protected by the Mantis Warriors Chapter of Space Marines, who maintained a number of bases on many of the planets within the cluster. The Mantis Warriors were members of the Maelstrom Warders alliance of Astartes Chapters that defended the Maelstrom Zone and were staunch allies of the Astral Claws. When the Astra Claws entered into a dispute with the Imperium of Man and eventually sought to secede from the Imperium so that the Maelstrom Zone's rich resources could be better used to protect its people from the predations of the Renegades and xenos within the Maelstrom rather than be distributed through the Imperial tithe, the Mantis Warriors stood with their allies. The Chapter joined the Secessionists' bid for independence as they believed an unjust Imperium was seeking to curtail the traditional autonomy of the Adeptus Astartes. This led to the civil conflict known as the Badab War in which Space Marine fought Space Marine on a scale not seen since the Horus Heresy. Some of the most bloody operations of that conflict occurred in the Endymion Cluster as the Loyalist Carcharodons Chapter assigned to the Maelstrom Zone to bring an end to the rebellion slowly and savagely brought the Mantis Warriors to heel, finally leading to their surrender at Endymion Prime. After the war, the Mantis Warriors lost their feudal claim over the Endymion Cluster and were required to undertake a 100-year-long Penitent Crusade during which time they could not recruit new Aspirants to make good their losses. The Chapter was considered to have been misled into rebellion by the actions of Lufgt Huron and thus not fully responsible for its actions.
The Endymion Cluster, the stellar region given to the Mantis Warriors to guard as one of the Chapters of the Malestrom Warders, was a region of Imperial space composed of 20 settled planets (6 of which were of major importance) that lay to the direct galactic east of Terra in the dangerous but resource-rich Maelstrom Zone of the Ultima Segmentum. The Cluster was surrounded on all sides by unsettled wilderness space and had never proven to be politically or economically coherent enough to be accepted as a fully-organised Imperial Sector since almost all of the human-settled planets within the Cluster were pre-industrial Feral and Feudal Worlds. The Endymion Cluster was located at an impressively dangerous astrographical position, with the horrors of the Maelstrom Warp Rift behind it and the xenos-infested Golgothan Wastes between it and the Imperium's core worlds. Because of the Cluster's proximity to the Maelstrom, it possessed few reliable Warp routes in or out of the Cluster and what routes did exist were often plagued by Warp Storms or daemonic and xenos attacks. The Mantis Warriors served as the guardians of the Cluster, having protected its people for thousands of years, though rather than establishing their fortress-monastery on a single world, they preferred instead to found a clutch of small bastions and watch-holds across the entire region, many of them cleverly hidden to better maximise the Chapter's penchant for guerilla assaults and hit-and-run attacks.
However, the largest and most important base for the Mantis Warriors within the Cluster was a region of the Death World of Tranquility III that the Chapter named the Valley of the Nine Winds. This base was primarily used as a training ground for the Chapter's Neophytes, most of whom were recruited from among the peoples of the Endymion Cluster, particularly the desert Feudal World of Tranquility II, and full Initiates. The Chapter's control over the Endymion Cluster had been vital not only for the protection of the people of these planets but also as an excellent forward operating area for the Chapter, which was tasked with cleansing the Golgothan Wastes and the Maelstrom itself of the vicious daemonic and xenos threats which were far more dangerous than the odd Ork pirate or human corsair. For their part, the people of the Cluster's mostly primitive Feral and Feudal Worlds believed the Mantis Warriors to be the embodied spirits of warrior insects sent by the God-Emperor to protect them, and their loyalty to the Chapter ran bone-deep because of the countless times these Astartes saved the people of the Cluster from the literal legions of hell, as well as the reality that most of the Mantis Warriors were recruited from their own tribes and cities. It was these people who during the Badab War would side so faithfully with their ancient protectors and kin, and they would pay a terrible price in blood when the Loyalist Carcharodons unleashed their final assault on the Mantis Warriors in the Tranquility Campaign.
- Sacristan (Frontier World)
- Khirab V (Dead World) - Khirab V was formerly a Fortress World of the Mantis Warriors until they were driven from the planet by the fierce fighting in the Badab War.
- Piety (Lost)
- Tranquility System
- Iblis III (Feudal World)
- Endymion System
- Sigard System
- Various void-based colonies and space stations
- Imperial Armour Volume Nine - The Badab War - Part One, pp. 6-7
- Imperial Armour Volume Ten - The Badab War - Part Two, pp. 14-15, 18-19, 69, 119