The Athar, also known as the Defiers, were one of the 14 factions that formerly lived in Sigil. The Athar believed that the deities of the multiverse were nothing of the sort- rather, they were merely very powerful beings that play the role of gods. Some Athar favored destroying and/or discrediting those supposedly false gods, while others merely sought to spread the truth about them.
The Athar were forced to leave Sigil after the Faction War. However, afraid to face the wrath of the powers they railed against on the Great Ring of the Outer Planes, the Athar moved to the base of the Spire of the Outlands, a place where not even godly power functions.