This came from a casual discussion on the glogosphere, but struck me as such a good idea that I had to write it down.
Many Cleric spells in several OSR rulesets have reversed forms, but it's often unclear how legal they are for PC's to employ. Sometimes they are interpreted as being limited to Evil Priest NPCs, other times that they might be permitted for use by PC's in extremis, or just that a nebulous warning that using them too much might result in alignment change. This is how I'm going to start handling the matter.
At any given time, a Cleric can be in one of two ritual states: Purity or Defilement.
While in a state of Ritual Purity, their spells heal, restore, and preserve, returning the fallen world to its proper state. All spells produce their normal effect.
While in a state of Ritual Defilement, their spells harm, break, and corrupt, reflecting human sin back upon the world. All spells produce their reversed effect.
At any time, a cleric in a state of ritual purity may choose to break the state, converting any memorised spells to their reversed forms. However, returning to ritual purity is harder than breaking it, typically requiring a return to a temple and spending a day in ablution and annointment - this explicitly does not require an Atonement spell to be cast nor is it accompanied with any sort of Geas or Quest; ritual purity is not the same thing as moral purity, and defilement is not punished, just rectified.
This presents an interesting logistical decision, particularly on a deep dungeon crawl - the party may encounter enemies potent enough to make a Slay Living or Harm very welcome, but once that bridge is crossed, then that Cleric is no longer able to provide healing, traditionally one of their most valuable party roles. At the bottom of a multi-level dungeon, with potentially dozens of encounter rolls between you and safety, this is no small thing. Bringing multiple clerics might be the meta strat, which is probably not too difficult - clerics are a pretty solid class, with few great weaknesses. I rarely ever actually see the stereotype of tables where nobody wants to play the healer; clerics are great at every level, and party composition tends to reflect that.
Settings where clerics of opposing alignments can be part of the same religion, or at least acknowledge the same cosmology - like Skerples' contextless OD&D, Deus Ex Parabola's Unfinished World, or WotC's Eberron - might be best suited to this idea. Some sects may even voluntarily choose to exist in a state of defilement for reasons both practical and theological.
Besides voluntary defilement, certain creatures or situations might induce it - getting energy drained by an undead, dealing peaceably with a demon, or getting extremely dirty, especially from something ritually unclean like gore, excrement or corpse-ash, would probably break your ritual purity. Maybe I'd allow a save for those sort of involuntary situations, but maybe also not - it might be amusing to figure out how the party gets their healbot through the sewer level.
It might possibly be prudent to introduce a magical item or even a mundane equipment item that allows Ritual Purity to be restored in the field, but make it either rare, expensive or awkward.
