My personal theory about the Dweller is that they may be some kind of servant of Oedon. After all, they live in Oedon Chapel, drop a Formless Oedon rune, and even look partially formless due to their cloak. Maybe they're tasked with finding a mother for Oedon's child, as Arianna's Third of Umbilical Cord description specifically mentions Oedon, suggesting that Arianna was impregnated by Oedon. "Every Great One loses its child, and then yearns for a surrogate, and Oedon, the formless Great One, is no different. To think, it was corrupted blood that began this eldritch liaison." The "corrupted blood" mentioned in the description is also likely referring to Vileblood, and it is heavily implied that Arianna is of Vileblood origin in some way. (She wears the Noble Dress found in Cainhurst, and the description for her blood states that, "A member of the old Healing Church would know that her blood is similar indeed, to precisely what was once forbidden.") Perhaps Oedon somehow learned of Arianna, and tasked the Dweller with seeking her out. The Dweller asks us to send any sane humans remaining to the Chapel, figuring that maybe they can trick us into doing their job for them. Or perhaps the Dweller really does just want to help people, and unwittingly aided Oedon in the process.