Upon returning from your journeys to the Professor’s apartment in Wroat, the Professor, the Countess (Kathi) Karrnathi, and a strange scarred man in dusty metal armor greet the party.
You are given a short briefing on what the three of them have pieced together from the information gained during your adventures, and they give you some important pieces of information.
Most importantly, it turns out that during the later parts of the war, Aundair began experimenting with a way to control Shifters, based on some of the research accomplished by the Church of the Silver Flame years ago during The Purge. Aundair planned to use this to regain control of the Eldeen Reaches and creating an army of shifters and lycanthropes to attack Breland, spreading lycanthropy to weaken the nation.
Baron von Vaughn contributed to this research significantly, and although it was abandoned partway through the war and he no longer had funding for his live experiments, he never fully divorced himself from the program of study. Kathy informs you that the Baron was contacted not too long ago to provide additional information and research to Aundair. Presumably Aundair wants this research to actively control shifters and the lycanthropic curse, although Kathy is not entirely convinced Aundair is behind the request.
It was around this time that your adventures took you to meet the Baron and retrieve a valuable magic item. The Baron was assassinated by a Nazathrune Rakshasa (a demon lord with powerful shadow and stealth abilities), who stole the identity of a Brelish nobleman arranged to be married to a Cyrian heir (Jane Vaughn) to accomplish this deed. A secondary objective for this Rakshasa was to steal the very magical artifact you were there to retrieve, but he clearly failed in that objective.
This artifact, if properly tuned, allows for fast and easy inter-planar travel, bypassing complicated and complex rituals which can be magically safeguarded against. By accidentally activating it in your retreat from the Mournlands, it was discharged and required re-activation and tuning to be useful again. When your adventures to do this failed (and resulted in an early confrontation with the Rakshasa whose plan you inadvertently foiled), an alternate method was found. Luckily, the gems you found in Thrane can be used to both re-activate and tune the artifact, and when used by somebody with the proper planer attunement once again allow for planar travel.
Now, the shadowy network of demon lords, including the individual you know as Duke Newcombe, plans to use the information gained from Baron von Vaughn and some additional incriminating evidence to prove that Aundair is actively researching a way to control Shifters in order to regain control of Eberron and attack Breland. This stands a very good chance of re-igniting the war if Breland is given this supposed evidence.
This is why the artifact is so important: the evidence is locked away for now in a secret place, to be exposed on Day of Shavalant, a Brelish holiday commemorating those lost in battle in the vicious Battle for Shavalant (996YK).
Advanced divination and scrying ascertained the location of the evidence: it is held in a high tower in the City of Shadows deep in the Demon Wastes. The City of Shadows is a mirror city; replicated stone-by-stone in both the Material Plane and Elemental Chaos of Kythri, it serves as a coterminous zone allowing for easier travel between the two planes.