Clerics can be Lawful, Neutral, or Chaotiic, even if they belong to the same sect. Neutral clerics aren't as committed to their beliefs; they are more like village priests, devoted to the community more than to an ideal. They cannot rise above 6th level unless they switch to Law or Chaos. Chaotiic clerics are heretics, religious deviants who believe they have found the True Way and that the others of their faith have it all wrong. If a heretic reaches the 7th level, they become the head of a heretical order, and at 8th level they become a heresiarch and can found their own monastery or temple to attempt to spread their personal vision. Thos who join the heretical order can be of any alignment, and if a Lawful cleric succeeds the heresiarch as head of the order, the heresy becomes an established sect.
Lawful and Neutral clerics can command the undead to depart (Turn Undead.) Chaotic clerics cannot. By default, all clerics, even the Chaotic ones, can cast healing spells. Reversed spells are not normally possible except to a 7th level cleric, and any use of a reversed spell risks corruption. (Make a reaction roll, on a Very Bad reaction, the cleric is corrupted.)
Clerics of less than 7th level can try to "research" reversed cleric spells or other spells not on the spell list. Clerical spell research is really more a matter of seeking divine favor by founding new shrines, instead of reading tomes of lost knowledge, but use the magic-user research rules for the costs, time, and chances of being blessed with a non-standard spell. These new spells are not transferable, but are unique blessings; each cleric must go through their own spell research to acquire the new spell for themselves. Using any spell that isn't a healing or utiliity spell carries the same risk of corruption as a reversed spell.
Corruption turns a cleric's alignment to Chaotic and strips them of healing power or the power to turn undead. They can still cast spells, but are not recognized by the dominant religion as clerics, but as Anti-Clerics or diabolists. Diabolists coerce or bargain with supernatural forces to increase their own power, for example to replace their lost turn undead power with a command undead power, or a curse power. The process of acquiring new powers that work like Turn Undead again uses the same research rules, although what diabolists actually do for their "research" is left to the imagination of the GM and players, and might be campaign dependent. A very medieval Christian campaign might have diabolists who defile hallowed grounds, while a more polytheistic campaign could have diabolical shrines.
A cleric can be an open heretic and possibly still be considered part of the church, but the heretic, as a Chaotic cleric without the Turn Undead power, might be tempted to secret diabolism to gain other powers. If not immediately discovered, diabolists can conceal their powers and continue to operate within the religious power structure. The eventual unmasking of a diabolist should be a great roleplaying opportunity.
To recap: Both Lawful and Neutral Clerics have the same powers, but Neutral Clerics can't reach 7th level, build a temple/monastery, or collect tithes until they change alignment. Chaotic Clerics can, but they do not have the Turn Undead power. Any cleric tempted by the promise of extra power becomes corrupted and turns to diabolism.
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.