Hydraphur is a Forge World located in the Segmentum Pacificus. It is the location for the main Imperial Navy battlefleet headquarters in its sector. Hydraphur is a prime recruiting ground for the Imperial Navy, with many of the prominent families on Hydraphur being connected to the Imperial Navy and various Rogue Trader houses. As a result of the influence of the Rogue Traders, Hydraphur is a commercial and manufacturing hub for the Imperium of Mankind and a person's financial connections are as important as their familiar one. This has lead to the Hydraphuran convention of adopting both a clan name (the business you are affiliated with) and a family surname (for example Lord Kalfus-Medell is a Lord of the Medell family, affiliated with the Kalfus clan). In addition to the Imperial Navy, the Ecclesiarchy and the Adeptus Mechanicus have strong interests and ties to Hydraphur. Law is enforced (as on most Loyalist Imperial worlds) by the Adeptus Arbites.
During the Plague of Unbelief in the 36th Millennium, Hydraphur surrendered to the rebel forces of Cardinal Bucharis, to its lasting shame in subsequent generations. After Hydraphur's return to the fold of the Imperium, Ecclesiarch Chye Balronas instituted on the world an annual religious festival characterised by fasting and penitent observance. Another legacy of Hydraphur's complicity in the Plague of Unbelief was the Administratum's attempt to curtail the power of the Imperial Navy in the system. Although the Hydraphur System was already a centre for Battlefleet Pacificus' operations, the administrative presence of the Ecclesiarchy and the Inquisition were increased, while the landholding naval families on Hydraphur were gradually relieved of their hereditary possessions. However, the success of the partition proved mixed: although the primary authorities on Hydraphur itself are the Administratum and the Ecclesiarchy, the Imperial Navy responded by tightening its control of the rest of the star system and the surrounding sub-sector, to the point where the Ecclesiarchy on Hydraphur found it necessary to use covert couriers to communicate with its outposts in other star systems. The Imperial Navy also formed close ties with the mercantile families on Hydraphur, to the point that their syndicate ties were actually regarded as blood family associations.
Hydraphur's political landscape is complex. The planet is nominally ruled by an Imperial Planetary Governor, however due to the number and influence of a number of aristocratic "great families" drawn from the most powerful commercial clans, this is mostly a ceremonial post with little real power. The space above Hydraphur and the rest of the star system is controlled by the Imperial Navy, who are, however, due to the Imperial Navy's complicity in the heresies of the Age of Apostasy in the 36th Millennium, barred from having any influence upon the governance of the planet itself. Also as a result of the Age of Apostasy, the Ecclesiarchy is charged with ensuring the piety of the population and it has a serious degree of political influence, which extends only as far as the upper atmosphere on Hydraphur itself. However, the Adeptus Ministorum is looked down upon as meddlesome and intrusive by the officers of the Imperial Navy. Inversely, the Navy is considered untrustworthy in the eyes of the Ecclesiarchy because of its involvement in the Age of Apostasy and its cliquish nature.
- Crossfire (Novel) in Enforcer: The Shira Calpurnia Omnibus by Matthew Farrer
- Legacy (Novel) by Mathew Farner
- White Dwarf 139, "Interstellar Travel" by Jervis Johnson, Andy Jones, Simon Forrest and Rick Priestley
- White Dwarf 140 "Interstellar Travel" by Jervis Johnson, Andy Jones, Simon Forrest and Rick Priestley