Hi all.  Loving DMing ToA so far.  My party is in the Tomb now and has started to be Haunted by the Sewn Sisters.  Understandably, they are quite distraught that all of their healers are slowly dying and no longer have any spell slots due to the Nightmare Haunting feature.  Heheheheh.  I had a smart player decide to wait until the haunting started and then cast Identify on the afflicted PC.  The wording of Identify states that "If you instead touch a creature throughout the casting, you learn what spells, if any, are currently affecting it." 

My in-the-moment decision was that the PC was unaware of anything afflicting their comrade as I don't consider the Nightmare Haunt to be a spell but instead a magical ability.  I just wanted to poll some of you and see what your thoughts are?  Thanks!

Technically, you are completely correct.  Whether you rule it that way depends on several factors, among them whether you believe it was intended to only work on spells, how much you care about the exact wording, and whether you want it to work or not. 

While, as Matthias says, you are technically correct, this sort of situation seems like a teaching opportunity. Most clerics that I play with tend to forget simple spells like Protection from Evil and Good can ward off abilities like these so that the party can sleep in a protected area. While they may not learn the true nature of the problem, their clever use of Identify should at least be partially rewarded with some extra bit of knowledge that maybe an evil presence is at work causing troubles for the afflicted players. Then maybe nudge them towards a solution and see if you can get them to start using more of the lesser used magics to protect themselves.

Thanks guys.  Almost exactly how I played it too...let them know there wasn't a spell afflicting the player...but something else is.  I'm going to wait till next long rest to see what they plan and perhaps will have one of the Nine Gods spirits give them a hint.  I'm thinking they may realize they have Tiny Hut available but haven't used yet...

Keep in mind that you can help them fail forward without directly volunteering info that Identify wouldn't give. You could remind them that Identify isn't exhaustive and ask if someone wants to try Detect Magic as well. You can also allow them to make ability checks to see if they know anything about night hags or the effects that Protection From Evil and Good can protect against. Nightmare Haunting is based on real world folklore to explain sleep paralysis, so it wouldn't be strange at all for their characters to have heard stories about being straddled by hags in a fantasy world where hags are actually real. That goes double for adventurers, since their line of work makes it much more likely they'll run into dangerous creatures.