Now, we already have an Exorcism spell on BG, for clerics and paladins. On BG it functions essentially as a single-target, weaker Chastise Spirits, (1d6 damage per caster level against enemy spirits) with the additional RP fluff of allowing the character to banish possessing entities. The easiest thing, of course, would be to just give them this spell as an SLA.Exorcism (Su): Starting at 13th level, as a full-round action, a spirit shaman can force a possessing creature or spirit out of the body it inhabits (for example, a ghost with the malevolence ability). To exorcise a possessing creature, she makes a class level check (also adding her Charisma modifier, if any), against a DC of 10 + the possessing creature's HD + its Charisma modifier (if any). If her result equals or exceeds the DC, she succeeds in forcing the possessor from the body, with the normal results based on its method of possession. A spirit so exorcised cannot attempt to possess the same victim for 24 hours.
However...
Mechanically, giving shaman Exorcism does not do anything. Chastise Spirits does more damage, and hits an AOE. And most shamans already RP being able to exorcise possessing spirits (and most DMs already allow them to, since it's core to the theme of the class). So, I have four suggestions:
Suggestion 1: Give shamans the Exorcise spell as an SLA, but let them use it more frequently than Chastise Spirits. Whether that's once per round, or on a short cooldown, or just as many times as Chastise but on a separate use, so if you have eg 5/day Chastise, and 5/day Exorcism, using one does not use charges from the other.
Suggestion 2: Give shamans the Exorcism spell as an SLA, but increase its damage for shamans (1d10 or 1d12 or something, so it's superior to Chastise against a single target). Have it use the same charges as Chastise/Weaken spirits.
Suggestion 3: Make shaman Exorcism a Dismissal effect instead (1/day or 3/day, I think). The reason for this is because, frankly, it's pretty disempowering when you see any random wizard/sorc, bard, or cleric just one shot elementals that would take you several uses of Chastise Spirits to bring down. Shamans already have very few things they can affect with their abilities, so seeing other casters being much more effective than you against one of them sucks (especially when you also see clerics arguing that incorporeal spirits should be changed to Undead so they can be affected by Turn Undead).
Suggestion 4: Give shamans a custom Exorcism SLA that is just a Will save-or-die, with similar DCs as Dismissal has, but only single target and only against enemy spirits (elementals, incorporeals, etc). This achieves a similar effect to giving SS Dismissal, but is a more conservative option.
Whichever suggestion, they should include fluff about being able to use it to force out possessing creatures/spirits.
Thank you for this consideration.