Turn Donald against the Spirit.

"Take care, Donald." you advise. "This... lady may not be all she claims."

The Beathan recoils slightly, taking the Lady of the Mountain's wrists in his hands, though he doesn't yet **** her touch away. Good, you think. Whatever this boon the spirit had promised was, you might be better off if Donald never comes into possession of it.

"In fact, something feels off about this whole thing. The way this... thing is sealed away, in such an isolated place, only the blood of your line able to open the door. It feels... demonic." you say, layering the mock concern thickly into your voice.

"Foolish king of a foreign land!" the Lady glares at you. "You know nothing of which you speak!"

"Did you ever hear of what happened to my father, Bryce?" you slur. "He met his end, no older than I am now, at the hands of demonic treachery! I beg you Donald, don't make the same mistake!"

That seems to do the trick. Donald pushes away from the Lady, taking a couple of steps back.

"Aye, you're right. We can't trust her!" the Beathan concurs. "Something about this doesn't feel right!"

"My Prince, please. This is nonsense. I harbour no ill will for you, or your people." the spirit pleads.

"To be fair, that's exactly what a demon would say." you point out.

"Enough! We need to decide what to do with her." Donald says finally.