Once inside, the Nephalem will find lots of corpses, and will note that the murders are too clean to be the work of the Reapers. The truth is revealed in the final room: the Nephalem enters just as Gorrel and his murderous thugss are finishing off the last of the noblemen.
"The Nephalem: —What is going on here?— Gorrel and the Nephalem(src)
Gorrel: —Revolution! It's time for our oppressors to fall. The king is in the city, but we have a trap waiting for him.
The Nephalem: —You plot and scheme at a time like this?!
Gorrel: —What? You're not with us? Then die with our enemies!"
Killing Gorrel and his thugs finishes the event. This event is required for the Long Live the King achievement.