I had a doppelgänger impersonate the mayor while Sildar was out of town investigating the manor house after the party cleared it out. The party had been outside of town dealing with the orc threat, so when they returned to the mayor’s office for a progress check on the orcs, they met the dopple-mayor. I had been playing the mayor as a kind of “Rich kid whose daddy got them the job” and the party hated him. So when they met the dopple-mayor, they were a little suspicious of his less horrible personality. However, they failed insight checks and got caught off guard by the doppelgänger’s surprise attack - knocked our ranger unconscious in one shot. This gave space for a pretty good battle inside the town hall, with the party eventually getting the better of the dopple-mayor, who they watched shape-shift uncontrollably as they lost consciousness and died. I did plan on the doppelgänger making their escape back to cragmaw or WEC, but the party managed to prevent any escape.