Hyades is a Jungle World in the Segmentum Obscurus not far from the Death World of Fenris that is a holding of the Navigator House Belisarius and is protected by the Space Wolves Chapter of Space Marines because of the ancient alliance between those two Imperial factions.
Hyades is unusually rich in naturally-occurring promethium, which is why the wealthy House Belisarius invested heavily in the exploitation of the fuel on Hyades and sponsored the settlement of an Imperial colony on the planet. Hyades is a hostile planet with large predators and an indigenous, ape-like, reptilian xenos species called "Reptos" by the Imperial settlers. The jungles also grow at an exceptional rate, requiring that they be burnt or cut back at all times to avoid them encroaching on the human settlements.
Even so, Hyades is not considered a Death World by the Administratum. Hyades has its own Planetary Defence Forces (PDF), which were drawn, at least in part, from mercenaries or retired soldiers of the Astra Militarum, including some raised on the Death World of Catachan.
As a result, the Hyades PDF was unusually professional. It was led by a mysterious man named Commander Cadmus who turned out to be one of the Fallen Angels of the Dark Angels Chapter of Space Marines, a ten-thousand-year-old Traitor to the Emperor from the time of the Horus Heresy.
The Hyades PDF had access to several Imperial armoured vehicles including Chimeras, Catachan Pattern Sentinels and a small number of Leman Russ Battle Tanks. The PDF took severe casualties during a battle between the Dark Angels, Space Wolves, and Thousand Sons Heretic Astartes as well as tribes of the primitive, native "Reptos" who were involved in collecting Space Marine gene-seed for the Thousand Sons. Though a disciplined force, the Hyades PDF ultimately proved to be no match for Space Marines, much less elements from three Chapters of them.
The Fall of Cadmus
Upon arrival on Hyades, Lady Gabriella Belisarius and her Wolfblade bodyguard, Ragnar Blackmane, a Space Wolf Space Marine serving as a bodyguard for House Belisarius as part of a penance for his Chapter, began their routine inspection of the planet's promethium quotas. Meanwhile, Interrogator-Chaplain Vargas of the Dark Angels had entered the Hyades System in search of one of the Fallen Angels, one of the ancient Astartes of the Chapter who had turned Traitor during the Heresy.
Unknown to Gabriella Belisarius or her Wolfblade, Commander Cadmus, an off-worlder in charge of the planetary defences, was the Fallen Angel. Commander Cadmus set in motion a series of events which led to a short conflict between an incoming patrol of Space Wolves and a Battle-Barge of the Dark Angels, the Vinco Redemptor. As the battle raged, Ragnar Blackmane worked out an uneasy truce with a Kill-team of Dark Angels to capture the ancient Traitor Cadmus and rescue Gabriella from his clutches.
As they closed in on their target it was revealed that the Thousand Sons Sorcerer Madox had masterminded the conflict between the Dark Angels and the Space Wolves as a diversion to collect Space Wolves gene-seed for a ritual to bring about the destruction of the Space Wolves Chapter, the Thousand Sons' ancient rivals.
Although Cadmus was ultimately defeated and a large Chaos Fleet was destroyed through a joint effort by the Space Wolves and the Dark Angels, Madox escaped from the planet and a massive Thousand Sons offensive began against the Space Wolves' homeworld of Fenris as the Traitor Legion sought to destroy their oldest rivals once and for all.
- Sons of Fenris (Novel) by Lee Lightner