If I had to involve Clerics in my campaign (nonexistent for a while, but who knows ?), here is a description I would make for my players (a summary of what I said previously) :
Clerics are religious warriors who are part of a very hierarchical order worshiping one only God. Once they proved their worth against Chaos, they are finally allowed to follow the Lamas' (or specialist in the Book) teachings. These scholars guide clerics through the five levels of the Book, each one bringing them broader spiritual knowledge manifesting itself by supernatural powers that reflect the cleric's deeper understanding of Creation.
But this exegesis is not unique: other interpretations, sometimes based on apocryphal texts, exist. Some say the world is the result of an unfortunate blend of darkness and light that will eventually split again. Others go even further by suggesting that the Creator is imperfect, as its creation, and that God has nothing to do with neither one nor the other. Lawful clerics are part of the mainstream whereas Chaotic are heretics, often rejected and fought by the Church. Patriachs, who inhabit strongholds, are the regional leaders of the Church wheras Heresiarchs are their counterparts.
Finally, Anti-clerics, who swapped their healing magic against a necromantic magic, gather together in sects worshiping (or even conjuring) demon lords. These cults, based in fortresses, are led by Evil High Priests who seek to overthrow the Church and impose their religion.
Rules modifications :
-Chaotic clerics can repel the undead.
-Clerics use spell books as magic-users.
-Limited to 5th-level spells.
They are treated as Clerics except for the following:
-Anti-clerics use Thief Advancement Table for experience only.
-Charisma is their Prime Attribute.
-Anti-clerics cast reversed versions of reversible spells.
-Turn undead ability is replaced by the Demon Control ability.
Demon Control : Anti-clerics can
conjure and/or control a demon whose true name they discovered in
their treaties of demonology. Anti-clerics roll on the Turning Undead Table to determine the success of their attempt. If the roll is successful, the demon is under
control (as the effect of a charm)
for 2d6 days x anti-cleric level. The dice roll must be kept secret.
Once this time has elapsed, the demon is free to do what it wants
(like disembowelling the anti-cleric) for a number of rounds equal to
its type. After that, it returns to where it came from. If the roll
is unsuccesful, the demon is free to do what it wants (like
disembowelling the anti-cleric) for a number of rounds equal to its
type. After that, it returns to where it came from.