Clerics devoted zealously to a particular philosophy can draw strength from it as followers of a deity can draw from their gods. Of course, "devoted zealously to a particular philosophy" is pretty much the basic description of a wide-ranging cross section of Sigil's population. Any faction would find clerics useful -- even the Athar have priests of the Great Unknown -- but clerics of particular gods or belief systems will either be namers only, or divided in their beliefs, unless they honestly consider their religion and their faction philosophy to meld into a single spiritual path.
More common than such theological balancing acts might simply be those clerics who draw their power fom the faction philosophy. Though these clerics would have their individual interpretations, most likely certain domains would be very common, and enterprising research theologians might well isolate from the faction's belief domains specific to those tenets. Such an achievement would certainly mark a philosophy as worthy of note, capable of organizing a cleric's spirituality and granting power in return for adherence.
I haven't found any -- even the faction descriptions of the Athar mainly suggest that an ex-priest can choose any domains he wishes -- so I thought I'd write some up! If this duplicates something obvious that I've missed somewhere... oh well.
These Domains would also be useful for arcane casters taking the Arcane Disciple feat from Complete Divine; for the Knowledge (religion) requirement of that feat substitute Knowledge (factions and guilds) if the game is using this skill from the PSCS.
The spells listed come from the 3.5 SRD, and sometimes the PSCS. In a Pathfinder game, remove the granted powers and select from among the spells listed those of roughly appropriate level to give clerics at the appropriate experience thresholds as "you can cast 1/day" abilities. The granted powers may occasionally be useful as 1st-level domain abilities.
The granted powers would doubtless be either "+1 caster level on domain spells," or faction feats from the PSCS, generally the ones with no other significant prerequisites. Unique granted powers are certainly possible, though. A few suggestions are given, along with other standard Domains that zealous belief in a faction philosophy might provide.
If there is interest I may submit these to the "Fractious Factions" section, especially after comments on the interpretation of the factions' philosophies. If someone would like to go through and pull out spells granted near a cleric's 1st, 2nd, 4th, 8th, 12th, 16th, and 20th level to build up Pathfinder domains, or make even better substitutions of Pathfinder-viable powers to grant, that would be awesome.
Athar Domain
Granted Power: Once per day, you may pit your belief against that of a cleric of a specific deity. Make opposed caster level checks using your caster level in the class that grants you the Athar Domain. If you succeed, your words echo in his heart, distracting him: for 24 hours any divine spell that cleric casts requires a Concentration check at a DC of 10 + your caster level.
Other possibilities: Faithless Blessing.
1st: Remove Fear (possibly Whispering Runes from the PSCS Chapter 5 if you use this spell.)
2nd: Gentle Repose
3rd: Remove Curse
4th: Dismissal
5th: Spell Resistance
6th: Banishment
7th: Resurrection
8th: Spell Immunity, Greater
9th: Mage's Disjunction
A cleric devoted to the Great Unknown often manifests the Knowledge Domain, and often an appropriate alignment Domain.
Believers of the Source
Granted Power: possibly Cumulative Life Experience, or Talents of the Past.
1st: Endure Elements
2nd: Restoration, Lesser
3rd: Heroism
4th: Polymorph *
5th: Commune
6th: Legend Lore
7th: Vision
8th: Planar Ally, Greater
9th: Shapechange
* The forms available through this Domain spell are limited, and vary from Godsman to Godsman; they believe the spell gives them access to the forms of past lives. It always grants the same form to any particular being, though these will differ among beings. It is theoretically possible, one supposes, for a Godsman to be on her first life -- in which case this spell would not function! The Believers of the Source would be extremely interested in such a person.
Priests of the Source frequently manifest the Creation Domain.
Bleak Cabal
Granted Power: Already Mad, Apathy, or the granted power of the Madness Domain from Complete Divine. Numb (a PSCS Dustmen feat) would also work.
1st: Sleep
2nd: Calm Emotions
3rd: Deep Slumber
4th: Crushing Despair
5th: Mind Fog
6th: Symbol of Fear
7th: Insanity
8th: Symbol of Insanity
9th: Weird
Bleaker Priests might manifest the Domain of Madness as well.
Doomguard
Granted Power: Destruction Embraced, or Sift.
1st: Entropic Shield
2nd: Shatter
3rd: Rage
4th: Shout (or Weight of Entropy from the PSCS)
5th: Flame Strike
6th: Disintegrate
7th: Destruction
8th: Horrid Wilting
9th: Implosion
Appropriate Domains would also include Chaos and Destruction.
Dustmen
Granted Power: The Dead Truce would be paramount if this is not already required in the campaign some other way; Dead Inside would work if so.
1st: Deathwatch
2nd: Ghoul Touch
3rd: Speak with Dead
4th: Death Ward
5th: Slay Living
6th: Antilife Shell
7th: Finger of Death
8th: Symbol of Death
9th: Power Word Kill
Death and Repose are natural Domains for priests of the Dead.
The Fated
Granted Power: Extra Skills or Self-Sufficient, perhaps.
1st: Unseen Servant
2nd: Arcane Lock
3rd: Vampiric Touch
4th: Secure Shelter
5th: Secret Chest
6th: Contingency
7th: Instant Summons
8th: Clone
9th: Summon Monster IX
Many Priests of the Fated would doubtless find reason to enjoy the Nobility Domain.
The Fraternity of Order
Granted Power: Intuit Pattern, regardless of other prerequisite (though what is a Guvner doing with less than Int 11?).
1st: Detect Law
2nd: Make Whole
3rd: Tongues
4th: Discern Lies
5th: Fabricate
6th: Analyze Dweomer
7th: Dictum
8th: Maze
9th: Wish
There are many natural choices for clerics devoted to the philosophy of the Fraternity: Law, the Mechanus Domain, Oracle, and Knowledge are all appropriate.
The Free League
Granted Power: Faction-Free, Free Willed and Slippery and all good choices for the Indep Domain granted power.
1st: Sanctuary
2nd: Remove Paralysis
3rd: Dispel Magic
4th: Freedom of Movement
5th: Break Enchantment
6th: Dispel Magic, Greater
7th: Ethereal Jaunt
8th: Spell Immunity, Greater
9th: Freedom
Zealous Indep clerics would very likely be attracted to the Liberation Domain.
The Harmonium:
Granted Power: Team Player.
1st: Command
2nd: Status
3rd: Hold Person
4th: Geas, Lesser
5th: Telepathic Bond
6th: Suggestion, Mass
7th: Hold Person, Mass
8th: Demand
9th: Hold Monster, Mass
The peculiar juxtaposition of attraction for the Community and Domination Domains is just another reason priests of the Harmony are looked upon oddly by observers.
Mercykillers:
Granted Power: Justice Blow or Know Lies.
1st: Detect Evil
2nd: Zone of Truth
3rd: Glyph of Warding
4th: Phantasmal Killer
5th: Mark of Justice
6th: Geas/Quest
7th: Forcecage
8th: Binding
9th: Power Word Kill
Whether a cleric of the Red Death is attracted to the Domains of Law, Inquisition, or Death tends to say something significant about their interpretation of the faction philosophy.
Revolutionary League:
Granted Power: Slippery
1st: Protection from Law
2nd: Undetectable Alignment
3rd: Nondetection
4th: Detect Scrying
5th: Mage's Private Sanctum
6th: Veil
7th: Refuge
8th: Screen
9th: Gate
Liberation would likely be a common domain for Anarchist clerics, though Inquisition bespeaks an Anarchist truly dedicated to penetrating the power structure's lies.
The Sign of One:
Granted Power: Dreamer or Powers of Imagination would be good. The ability to cast Conjuration (creation) spells at +1 caster level, enjoying the same synergy with Artifice and Creation as those two Domains already do, would also be interesting.
1st: Silent Image
2nd: Eagle's Splendor
3rd: Helping Hand
4th: Minor Creation
5th: Major Creation
6th: Programmed Image
7th: Limited Wish
8th: Antipathy
9th: Wish
Artifice and Creation would also be good Domains, as mentioned above. Nobility is also a possibility.
Society of Sensation
Granted Power: Alertness, Extraordinary Sense, and Sensory Touch are all possibilities.
1st: Identify
2nd: See Invisibility
3rd: Clairaudience/Clairvoyance
4th: Arcane Eye
5th: Prying Eyes
6th: True Seeing
7th: Arcane Sight, Greater
8th: Discern Location
9th: Foresight
Knowledge, Oracle and Inquisition are both useful for the inquiring Sensate.
Transcendent Order
Granted Power: Cadence Whispers.
1st: True Strike
2nd: Cat's Grace
3rd: Keen Edge
4th: Dimension Door
5th: Teleport (Divine Agility from Complete Divine appropriate)
6th: Find the Path
7th: Teleport, Greater
8th: Moment of Prescience
9th: Time Stop
Clerics of the Cadence would doubtless frequently manifest the Celerity, Luck, and possibly Oracle Domains.
Xaositects
Granted Power: Scramblespeak was the 2e defining ability of the faction. Unlikely Event could also be an interesting granted power.
1st: Detect Chaos
2nd: Blur
3rd: Blink
4th: Confusion
5th: Feeblemind
6th: Mislead
7th: Word of Chaos
8th: Cloak of Chaos
9th: Storm of Vengeance
Common Domains for clerics of the Xaositect philosophy would certainly start with Chaos and Luck, with the Limbo planar Domain as another common choice.
An interesting variant on the Xaositect Domain would be the following: at the time of character creation, the GM rolls randomly to choose one SRD spell of the appropriate level from among all spells he decides are reasonably available: at the least, cleric and sorcerer spells not in conflict with the player's alignment. These spells form that player's Xaositect Domain. The character is aware of what each of the spells is or could be.
This is really nice.
BoGr Guide to Missile Combat:
1) Equip a bow or crossbow.
2) Roll a natural 1 on d20.
3) ?????
4) Profit!