when can i fight in the arena in game of thrones?

Answer 1

The arena is only available in Chapter 8, so make sure you do it during this chapter....

You have to complete your quest in the sewers, leave the sewers and then come back in....You'll notice the arena has filled up with a bunch of onlookers and a man named Ossifer you can talk to.

Speak with him to start a series of fights....and try not to die.  ;)