How about saving the Hags daughter? Obviously she would be a changeling. Make her young enough that she is just starting to feel the call. Make her a neutral sorceress who is in danger of heeding the call and turning into a hag herself. This gives the paladin the opportunity to save her not only from the danger she is in, but also her heritage. Play her as a basically decent person with some with some tendencies towards evil due to her mother. The paladin’s job is to save both her body and her soul even though he may not realize it at first. Don’t reveal the fact she is the hags daughter right away. This tests the paladin on multiple levels but is not a dick move.
Oh, did I not mention that the NPC the cleric is playing meanwhile is a changeling witch from the town? She's going to be part of the hook for the followup adventure we're doing currently in 4 levels. lol
I like this
For further detail. I wanted to avoid a pure arcanist to avoid stepping on other player's toes too much. Almost went oracle, but decided I liked the witch a little bit more, and will allow the cleric's PC to branch his knowledge a bit. Changeling was decided since there is some Dark Tapestry stuff in the mix, and wanted something weird but not too weird.
Then I started coming up with the scenario... Hags were something they're not supposed to run into during the course of their extended campaign. I wanted something the paladin wouldn't simply be able to "DUR HUR Smite". They're way outside the scope of the party to take on. CR 13 together? Party is APL 4. I am very much of the opinion that "It's evil and thus auto-killable" with intelligent creatures is just as evil as the creature is. Especially since the Hags are not known to currently be messing with the town (whether that's because they've recently arrived or have been cowed by the evil the players are eventually building to deal with isn't decided, and outside my current scope.)
As for the paladin working with the hags. Working with evil for a greater good or to put down a greater evil is within the realm of this IMO. Saving the cleric from death is unrepentantly good. Cleric of a good god, working unknowingly against a "Super smite" villain. So, saving greater good against greater evil to correct paladin screw up. IMO walks the paladin line, but doesn't go over.
I also disagree this is not exceptional. A close friend, story built relationship with another PC cleric will die and be outside the realm of raise dead without again the said hags (Only things close enough without abandoning the region to perform the recovery). I'm ok with this as the GM. Should make for an interesting story.