To get this you will need to play as a female character (males get the Handmaiden). After you get off Telos you will unlock four different planets: Korriban, Nar Shadaa, Onderon, and Dantooine. The Disciple is on Dantooine. Go there and enter the Khoonda administrative center and talk to the administrator. After a lengthy conversation she will give you access to the Jedi Enclave sublevel. Travel there and the Disciple will be in the room at the opposite end of the map. Battle your way through the laigreks to him (or use a high stealth character and sneak there) and talk with him for a while and he will offer to join you. You can't refuse him and you will unlock the achievement.