You see a distinguished-looking bearded man wearing a purple toga. He is preceded by 12 attendants (called lictores), each of whom carries on his shoulder rods bundled with an axe and wreathed with laurel (called fasces).
To salute him, type /greet emperor (don't forget the slash); to show that you are displeased with him, type /insult emperor. He will communicate with you if you click activate, though emperors are notoriously unpredictable. You can always shut him up by clicking hush.