This quest is not available in game.
Warden Alturas wants you to enter the Violet Hold and put an end to the blue dragon invasion force. You are to report back to him once Cyanigosa is slain.
Here is the prison key.
My men and I will stay here as a last line of defense in the event that you are unable to overcome the assault.
Be sure to speak with Lieutenant Sinclari. She'll provide you with access to the prison's defenses.
Hurry, <race>. There is little time. The few I left inside won't last long without your help. Report back to me once the invasion forces have been successfully dealt with.