When "Into the Pitt" is unlocked, you must find a slave called Midea.
Talk to her and she'll give you a job.
Then go to the Steelyard and find 10 steel lingots.
Give them to the man who opened you the door.
Go talk to midea again.
She'll tell you about a way to win your freedom.
Go to the arena, talk to Faydra and win the 3 fights.
Achievement unlocked !