Join the Khans

Join the Khans is a side quest in Fallout.

Detailed walkthrough
Upon entering the main building of the Khans' camp and talking to Garl, the leader of the Khans, tell him you want to join, and "kick some serious ass" (choosing any other dialogue option will result in the Khans attacking you). Garl will then tell you that he is tired of his female slaves, and that you must prove yourself by killing them.

Doing so is easy, as they cannot fight back but will simply run away. However, you must kill them before they leave the camp, as killing them outside the camp will cause the Khans to turn on you. Once they have been killed, Garl will name you a 'friend of the Khans', and grant you free reign of the camp.