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Recruit Charon is a mini-quest in Fallout 3

Accepting the Quest
If you speak to Ahzrukhal in The Ninth Circle in the ghoul town of Underworld, he will tell you that he holds the contract of employment (read: slavery) to a ghoul named Charon (also located in The Ninth Circle). Charon is bound to work for whomever holds his contract. Ahzrukhal also tells you that he dislikes having to compete with the other beverage outlet in Underworld, Carol's Place. He proposes that if you were to kill Greta, who works there, he would be willing to give you the contract to Charon.

Killing Greta
Remember that if anyone sees you kill Greta, they will become hostile to you, so the best time to kill her is when she goes on her smoking break just outside of Underworld in the Museum of History lobby area. Even if nobody witnesses you kill her, you will still receive Negative Karma.

Reward
After you murder Greta, return to Ahzrukhal to report your success. He will stick to his word and give you Charon's contract. After you tell Charon that you now have his contract, he will walk over to Ahzrukhal, who will think he's coming to say goodbye. Instead, Charon pulls out his shotgun and kills Ahzrukhal.

Charon will faithfully serve you regardless of your karma. You can have him carry items for you, shoot enemies, use melee attacks, and (surprisingly) use power armor, etc.