Mobidium

Mobidium was a massive Oceanic Death World located within Segmentum Tempestus. It was home to the Novus Krakens chapter of Space Marines before its devastation in 838.M41. Its oceans were once the home of a variety of highly dangerous sea monsters. The history of Mobidium is long and complex, having many tales of incursions from the daemon, most of which are lost to time. Two moons orbit the once pristine blue world, Aerwyne and Antkutmeri.

Unfortunately, with the loss of the Fortress Monastery of the Novus Krakens during the Fall of Mobidium, not much is known about this world to the greater Imperium, though a very small handful of records make reference to three gargantuan Sea Monsters known as the Guardians of Mobidium, who took the forms of the Ancient Terran Kraken, Leviathan, and Hydra.

History
While the Imperium knows little about the world and the three wayward kinsmen of the Adeptus Astartes that once called it home, the in-depth records of the Penumbral Hydras renegade chapter contain a complete history of the world.

The oldest recorded information regarding the culture of Mobidium is a tale about three champions of humanity who stood against a tide of otherworldly hellish creatures from the depths of the oceans. In this epic story without a solid date, the three warriors were told to have defeated the three leaders of the horrors from the deep, and became the monstrous Guardians of Mobidium. Analysis of this story reveals to any being with knowledge of chaos would know that it was daemons that assailed the lone world, and so a much more plausible reason for the creation of the Guardians could be extrapolated; that the three failed to defeat their foes and so pledged their servitude in order to protect the world. Whichever the case, the Guardians proved able to ward away the daemons somehow in some capacity.

The three Guardians of Mobidium were given the names Krakonicus, Kenrste, and Etflerien; the Kraken, Leviathan, and Hydra respectively. For millennia a great respect for these protectors shifted into worship, taking these monsters to be protectors that kept the world safe. When the Imperium arrived during the Great Crusade, the people resisted vehemently in the eradication of religion and the adoption of the Imperial Truth. These more religious individuals survived on the oceans, living their lives in perpetual piracy for their beliefs. Unfortunately, the Horus Heresy began before the Imperium could secure the eradication of religion on the world. So for several millennia the hunt for the religious pirates stagnated until missionaries of the Imperial Creed arrived on the world in the 33rd Millennium.