Quickleaf
Legend
I’m familiar with the adventure and the NPCs involved, but I don’t know if you’ve made significant changes to things or not, so this idea may or may not work.
Yeah, since you're familiar with Tomb of Annihilation, I've made a couple changes to how Xandala appears there. In ToA Xandala's motive is kinda divorced from the main story. She's an outsider. Instead, I tied her draconic bloodline to two elements in Chult: a black dragon called Mergandevinasander who I have as once inhabiting the Aldani Basin & to the Sewn Sisters. I also made her a sort of renegade Red Wizard. Thus, she serves to foreshadow those elements. Still trying to iron out the kinks in her backstory.
Can Xandala possibly convince the party that the NPC has come under the sway of the artifact, and is no longer in control of his own actions? Perhaps she can convince them to subdue the NPC, and transport him to a location that holds the key to breaking the curse....a garden that contains the “Summer Bloom”, a rare flower that can restore the NPC’s right mind.
Of course, instead she leads the PCs to the medusa’s garden where they’re ambushed by both the goblins and the medusa.
Maybe that could work?
Yeah, that's totally possible. But the goblin paladin PC's zone of truth kinda kills that; all he has to do is cast it and pose the question "Are you lying about him being under the artifact's sway, yes or no?" Sure, Xandala can refuse the spell, but that immediately makes her suspect. The PCs are already wondering what's up, whether the missing NPC (Artus) is mistaken and indeed has a daughter, or if Xandala is mistaken or lying; while discussing their course of action, zone of truth was brought up.
I'm not saying that zone of truth rules out all such scenarios – the last time he tried to use it, his line of inquiry proved inconclusive because he was investigating a murder in which the murderer wasn't directly responsible and had made another innocent culpable. However, the situation with Xandala is one of those cut-and-dried ones that zone of truth is intended for.
Although, since the NPC is actually a doppleganger dsiguised as the NPC, I don’t know how long that would remain a secret from her....
Right, as soon as Xandala tries to cast dominate person on the doppelganger, the spell fails because it's a monstrosity and not a humanoid. Perhaps her first move is to convince the goblins to steal the ring from "Artus" while he sleeps.
Does the doppleganger have the artifact in question? I don’t know if this was explicitly stated in your descriptions above.
The doppelganger does NOT have the artifact. The doppelganger has some skills beyond the MM doppelganger, including the sphinx's Inscrutable Trait, Survival proficiency, skill with a longbow, and Spellcasting (wizard) allowing it to cast message, minor illusion, blur, illusory script, Nystul's magic aura, and whispering wind (homebrew spell). Part of its routine is casting Nystul's magic aura on its ring and itself to make the ring appear to be the Ring of Winter and itself appear to be under the magical effects of the ring. The party's bard PC subtly cast detect magic on the doppelganger, and learned what the magic aura wanted him to learn.
It's a bit convoluted, but the doppelganger met the missing NPC (Artus Cimber) in the jungle, sympathized with his plight & read his mind, planning to masquerade as the famed explorer Artus Cimber (for the doppelganger's own ends). Well, Artus realized what was happening, but the Ring of Winter is slowly making him coldly pragmatic, so rather than warn the doppelganger not to attempt the masquerade due to the many enemies he has, Artus let the doppelganger do its thing, hoping to use it to flush some of his enemies out into the open. The doppelganger is an agent of one of the Beggar Princes of Port Nyanzaru that I made up known as the Invisible King who seeks to liberate elementals (esp. air elementals) from binding magic, and has made an uncomfortable arrangement with Withers (from the Tomb).