Tetragramatton

Tetragrammaton is a paradise world located in the Ultima Segmentum that serves as an Adeptus Astartes homeworld of the Exalted Seraphs Space Marine Chapter.

Background
The Exalted Serpahs' homeworld of Tetragrammaton is a beautiful paradise world located just north of the nearby Realm of Ultramar. It is a seemingly peaceful world, but only the nobles and important members of the Ecclesiarchy live a luxurious life, while the peasants are relegated to a life of backbreaking labor. The north and the south poles of the planet are inhabited by abhuman beastman and other Chaos mutants. They usually target cities with high populations, wreaking havoc with wanton destruction and slaughter.

Before the arrival of the Imperium during the Great Crusade, the planet's human population was on the brink of extinction due to constant beastmen attacks, but then the people were saved by an army of knights, led by a mysterious woman, simply called, the Lady. However, the Lady said that the people must serve as her servants forever and sacrifice witches for her so that they could prevent the beastmen herds from attacking again. Some Imperial scholars think that it is a mere allegory for the Great Crusade, and that the army of nobles were simply Space Marine Legionaries bringing Tetragrammaton into the Imperium. However, new archaeological digs of the planet discovered some forms of armour from the Dark Age of Technology, and there were statues of the so-called Lady pre-dating the arrival of the Great Crusade. When the world was brought into the fold of the Imperium, the cult-worshiping the Lady, known as the Temple of Her Lady, went underground to continue her worship, in defiance of the tenets of the so-called 'Imperial Truth'.

During the appearance of the Imperial Cult in later centuries, which would later be recognised as the Imperium's official state religion, the Temple of Her Lady used this opportunity to disguise themselves as a sanctioned sect of the Imperial Cult, and depicted the Lady as an Imperial Saint instead of a goddess. If someone were to join the cult, he or she must be born from noble blood. Almost all the Exalted Seraphs are part of this cult. However, it is implied that the Lady isn't even human but a delusional Aeldari, who was once a high priestess of Lileath, the Goddess of Dreams. She believes that she is the reincarnation of her patron goddess and uses the knights and peasants to sacrifice psykers to keep her from aging, and tricks them into thinking that she is an actual goddess.

The Temple of Her Lady isn't the only obscure offshoot of the Imperial Cult. There is also the heretical Church of Singularity - a cult that worships the Emperor - not as a god, but as an enlightened being who will help Mankind to transcend to holy beings. Their motto is, "Enlightenment through order." They are supposedly derived from an ancient Terran religion known as Buddhism. The cult is currently being actively hunted by the Exalted Seraphs due to their connection with their former Chapter Master Camael, and they were discovered to have worshiped a pantheon of unknown entities similar to the Chaos Gods, known as the Lords of Order.