Once you get the the The Castle of Light zone, Lux will intercept you asking you to retrieve his key from the The Unenlightened zone.
The entrance is in the castle of light zone, so head over there and pick up Lux Sanction Key.
Important!! Once you get back, do not give Lux the key back. Instead go open the sanction/basement.
Once you exit the basement Lux and his hired muscle will turn on you. Defeat him for the achievement.
Important!!!! I got hit with a bug that when I left the zone after killing just lux and came back the enemies were no longer hostel, so if you are playing on iron mutant try to take lux out before leaving just to be safe.