SideQuest - The Crucible
This is an optional arena where you can train and win some unique equipment as well. This place is available in your map however you can only access it by redeeming the code that came along with the game. Consult the back of manual for this.
It's setup is similar to the Arbiter's Maze where you need to fight waves of enemies which get stronger as you progress. To access higher waves, you need to meet some requirements or milestones in the game. You'll be notified when you unlocked a milestone in the form of Tarot cards that will be sent to your mailbox. Access any Tome to retrieve this “access cards”.
Defeat the Guardian
Defeat the Wailing Host
Wave 101 (Wicked Killington)
Beat all waves in a single visit
Defeat Wicked K.