Kill Emperor Dagran Thaurissan.
Emperor Dagran Thaurissan has total control over the city from his throne ahead. If we are to stop his armies, we're going to have to end their emperor's reign.
You'll need to get past his personal guard, including the giant Magmus, to reach him. Move as quickly as possible - if word gets out before you finish, the entire city will descend upon his throne.
Note that the doors across the Lyceum will only open once you've lit the braziers with torches from the flamekeepers. Good luck.