"Third headstone on the right and six feet under, that's where you'll find Stellan." --Devon
While playing the role of "The Stranger" in Eggley's traditional ceremony to Killian, Stellan was killed by the Collector, to whom he owed a debt. Devon explains to the party that Stellan died without emplacing the festival's sign for the townspeople to return to Eggley, and they had therefore settled elsewhere, leaving Eggley abandoned.
In Chapter 6, the party learns that the maps for the mines under Sarth had been given to Stellan to assess how much Sarth's Abbey could be taxed in light of its connection to the mines. Since he had been Nivek's contact in Eggley, Stellan's house there is a Kingdom treasury office and can be opened with Nivek's Key.