The Chapel of Vanity, also known as the Cathedral of Light or Cathedral of Vanity, is a small dungeon accessible from the Library of Fate. It only consists of a single big room initially inhabited by the Veil Piercer and Nuns of Vanity; once the Crystal Ball is destroyed, the true boss The Prophet will spawn.
"How much suffering, mortal, does it take before you lose your grace? Let me show you..."
Angels and Demons, opposing sides of the same coin, locked in a never ending tug of war. One day, however, that coin briefly landed on its edge... The archangel Inarius and the demoness Lilith had long awaited an opportunity to break the shackles binding them to the Eternal Conflict and, when the rebel seraphim stumbled upon the Worldstone, fulcrum for the shaping of reality, time was ripe. The two lovers hid it from their kin and with it forged Sanctuary, a refuge and eventually home to their offspring, the nephalem. Their weak human descendants retained the latent potential of their ancestors and, through magic arts, one day experimented with the summoning of demons. These acts brought Sanctuary's presence under the observant eyes of the Prime Evils, whom promptly established the Cult of the Triune with intent of subjugating humanity under their rule. Inarius, to ward off their influence, created the Cathedral of Light, upholding the tenets of Heaven. This ignited the start of the Sin War, culminated with the confinement of Inarius in the Burning Hells under Mephisto's keep. The Lord of Hatred, after centuries of torment, managed to bend Inarius' spirit, collecting a new pawn for his schemes.