The Source is a mysterious concept that seems to be key to the existence of Mania. Nearly nothing about it is known, but it is hinted to be at the center of the Deep Mania and the edge of the destroyed city of Maria inside BR-000. The Underground's goal seems to involve reaching the Source, and Paradeisos is able to approach the source, though not quite reach it, through the Hush system and its Observatorium. It is said that by feeding on Black Rings, the Shackles can become a pathway to the Source.
The Corpus of BR-001 tells the Chief that the two of them "are the Source". It is possible that the Source thus refers to the Great Existence itself, of which all Corpuses may be a component part. After witnessing the Source, Hestia claimed that it was an "indescribable... twisted, disgusting" thing and "the real god".
The Source may also be referred to as the "Ultimate End". Mantises in the Nightmare Space profess a desire to "embrace the Ultimate End", which requires them to move closer to the center of DisSea. This suggests that the Source may be the Perishing Star itself or something contained within, as it is the source of Mania in the post-apocalyptic world.