This quest is a sub-quest of Welcome to the Peacock Club and is initiated by examining the Service Entrance Key, Truth to the House of Truth. This key is dropped by a Palace Sweeper which spawns at ( -230, +160, -131 ) and then paths towards the arena. The Court the sweeper belongs to is random so you may have to go through a good number of them to get the correct key.
You must then sneak into the Court of Truth using the service entrance on the side of the Court. Once inside you must clear the mobs in the zone to get three keys and then work your way downstairs to obtain the banner.
This will spawn a guardian who is blocking your exit. You must kill him before you can exit the room. You must exit through the service entrance NOT the front entrance.
You have 10 minutes to complete this entire entrance.