Winds of Chaos

I’m writing a campaign where it would be useful for a powerful NPC to be able to summon and compel a ghost for their specialist knowledge.

I’ve tried searching through the different editions and supplements I have, but I can only find a prayer for priests of Morr that lets them ask a single question.

Happy to make it up, but is there a canon way of doing it?

Not really. I would just make up something, probably either a religious or magical ritual.

The 2e book Realms of Sorcery and/or Tome of Salvation can probably provide some good inspiration.

amythest wizards are supposed to be able to do that, although that sort of thing would fall under a ritual rather than a spell. not all bad, part of the adventure could be settng the stage for the ritual (gathering the remains needed, special regeants, scouting a suitable area to make sure it's safe-) which can be fun.

unfortunately rituals have never been very well described (partly to keep the PCs from abusing them) so you'll have to wing it a bit based on how much work you want this to be. maybe make it have to be a special day to work to keep them from abusing it....

Lovely! Much obliged. That'll do