The Athar are one of the Factions that contest for the minds and souls of Sigilites in the Planescape setting for Dungeons & Dragons. A weird mixture of atheism and Buddhism, with a touch of monotheism, the Athar are united under the single, simple belief that the various gods of the multiverse are not "real" gods, but rather merely extremely powerful entities who are exploiting the gullible denizens of the planes; they do believe there is some kind of "true" divinity, the Great Unknown, which lies beyond anything that mere mortals can hope to perceive - at least in their present state.
Needless to say, they're not a very popular faction in-universe, for various reasons; the Harmonium likes to use religion to promote conformity and so disapproves of someone attacking the cornerstone of religions, whilst the Mercykillers are leery of accepting the idea of "the Gods are Frauds!" because that'd mean that Justice would demand they be punished, but they're too strong for the Mercykillers to stand a chance, for example. About the only faction that really likes them are the Believers of the Source, since their philosophy of "ascend to divinity by unleashing your innermost potential" is fairly close to the Athar's own beliefs.
Joining up with the Lost is as easy as walking into their base at the Shattered Temple and asking to sign up. The Athar know they need numbers, so they're not very picky. So long as you're willing to bring in the three articles (sacred weapons, holy texts or divine magic items) of a power that need to be destroyed as part of the initiation rite, that's all you need to do. Of course, getting up the ranks is more challenging; you need to have a significant victory over either a power or a power's proponents in order to move up in ranks. And if you want to become the new Factol? You got to defile an active chapel or temple, which few berks survive doing.
Members of the Athar are immune to various Divine Enchantment spells; abjure, augury, bestow curse, divination, enthrall, exction, holy word and quest all fail outright if they are used on an Athar.
Those who reach the rank of Factotum (or "Athaon" to the Lost) also gain a general +2 bonus to all saving throws vs priest spells cast by powers worshippers and the spell-like abilities of creatures that directly serve a power. Aditionally, when sufficiently large groups of high level (4+) Athaons gather, they can actually produce a Banishment effect; when four or more Athaons encircle a power-serving petitioner, planar creature or proxy, they can spend a full round action to force the encircled being to pass a save vs. spell or be banished to their power's realm for a year, a month or a week, depending on if they're a petitioner, creature or proxy. They do their best to keep this ability under their hat, because they don't want it to be well-known that they can banish certain planars just through sheer force of will.
Factors (9th level or higher Athar) gain a permanent Obscurement power which makes them immune to pretty much all Divination spells and spell-like abilities cast by power-aligned casters, even those used by the powers themselves, forcing the caster to pass a save vs. spell in order to make their spells work.
Finally, the Factol is the only Athar who can use the power of the magical Bois Verdurous tree that grows in the center of the Shattered Temple. By touching it, the Factol can cast any priest spell he wants, as long as he wants, though such spells can't reach beyond the temple itself. Or he can pluck either a fruit or flower from the tree, which becomes a 1-use enchanted item bearing a single priestly spell of the Factol's choice. Anyone else who tries to touch the tree takes D12 damage each round, has all their magical items drained of power, and loses all their memorized spells (if applicable).
Knowing they're on shaky ground as perhaps the least-liked faction of all prior to the Faction War, the Athar focus mostly on attempting to score conversions through propaganda, with a particular emphasis on harassing clerics and exploiting religious layfolk suffering crises of faith. After the Faction War, they are forced to flee to the base of the Spire in the Outlands, where they can still be shielded against divine magic now that they've been kicked out of Sigil.