The Obsidian Cathedral is the darker inverse of The Marble Cathedral, where the three Illiadian Deities of Shadow, Sh'ther Kei-Sihr, Tahvihsa and Prethao, but also their servants and prophets make their home. Compared to The Marble Cathedral, The Obsidian Cathedral isn't exactly a good place to be at for a non-resident. Its halls are teeming with Shadowspawn of any power-level and sapience-level, most of which thankfully aren't as aggressive within the presence of Sh'ther as they usually are, but it's also generally said that it has a bit of a corrupting effect on the people that visit. So far, no negative consequences to oneself have been noted by any religious scholars, but generally, The Obsidian Cathedral is to be avoided.
The hierarchy of the Obsidian Cathedral is the same as the Marble Cathedrals', with Shadowspawn and Awakened Shadowspawn replacing Divine Spirits and Awakened Divine Spirits.