The Athar are a faction of philosophers in the Planescape universe, created by TSR (now Wizards of the Coast/Hasbro) for the Dungeons and Dragons Role-Playing Game. Based at the Ruined Temple in Sigil, they declare that all the many powers and deities of the Multiverse are false gods, and that the only being worthy of worship, if such exists, is the unknowable. Some of their number dedicate themselves to this force, known as the Great Unknown, although others spend their time decrying the lies of the priests of the local temples. They are especially fond of the Astral Plane, since the stone corpses of dead 'gods' reside there, a powerful testament to the mutability of the powers.
The original idea of the Athar appears to have been as strong atheists in a world where religion appears to be a matter of 'scientific' fact, much like Abraxas the Agnostic in Terry Pratchett's novel Pyramids. However, as the setting developed, the idea of an unknowable ultimate divinity appeared, and their leader, Factol Terrance, appears in The Factol's Manifesto as a prophetic figure, with slight similarities to Muhammad, proclaiming the futility of polytheism in the face of One Truth of Dvinity.