The Church of the Unconquered Sun was an occult religious organization that worshipped the Sun-in-Splendor and its Names: the Madrugad, the Meniscate, and the Sun-in-Rags. In the Third and Fourth Histories, the Church was extremely powerful. The Church’s priests were all male, but would allow women to become priests if they partook of rituals that would allow their ceremonial passing into manhood. Especially sacred rites of the Church would be performed in the Concursum, the deepest chambers of the Mansus accessible via the Spider's Door. This implies human sacrifice may have been a core facet of the Church’s proceedings.
There is no information on how exactly the Church came into being. Looking at its parallels with Roman cult of Sol Invictus, which was founded in 274 AD, it's possible to assume that the Church originates from that time as well.
At some point before the War of the Roads, the Sisterhood of the Knot influenced the ideologies of the Church. Some within the Church’s ranks were appalled by this paradigm shift, Hieronymus Pseudo-Hypnerotomachus being one such critic. The Sisterhood’s influences may have touched even the Church’s most sacred rites - those performed in the Concursum within the Mansus. According to 'The High Mysteries of the Innermost Chamber of our Church Solar', these rites include corollaries for the non-solar Hours: the Red Grail, the Ring-Yew, the Horned Axe, and the Thunderskin, which were worshipped by the Sisterhood, what can be a result of that influence.
The Church of the Unconquered Sun, alongside the Sisterhood of the Knot, mounted a defense of the European Continent against the invading armies of the Sovereigns of the Leashed Flame in the War of the Roads. It's possible that Imperishable Legions mentioned in the 'How the End Will Begin' are the Church's forces.
The Church was weakened severely during the war. The Intercalate, which came two years after the war’s end, succeeded in destroying the already-fragile Church. The death of the Sun-in-Splendor caused a schism in the organization, which dissolved the bonds of its power and effectively ended the institution. It's said that it was driven underground in many Histories, however, it's unclear whether it happened prior or after the Intercalate.
1. Sunset Passages 2. Read 'Sunset Passages' 3. Read 'The High Mysteries of the Innermost Chamber of our Church Solar' 4. Against the Sisterhood of the Knot and... 5. Read 'How the End will Begin' 6. Sunset Passages (untranslated)