Your character is a senior at the Sith military academy, where the military leaders of the sith empire are trained. She is attractive, cunning, and skilled, and constantly showing up her classmates. When she graduates, most believe that she will have her pick of any assignment in the empire.
A half dozen students at the academy, including your character, are selected for an elite training program, where they will spend time working as junior officers on a young sith lord's personal command ship. The assignment is a dream come true, as it will give each student a chance to impress a sith lord and earn his favor. Unfortunately, this isn't to be. Everything is going fine until a mission to capture a jedi spy goes horribly wrong. The spy escapes with critical intelligence and the sith lord on the ship is made to look like a fool. Everyone knows that someone will die for this.
Desperate to keep themselves alive, the other academy students all work together to frame your character, making it look like the operation fell apart because of her recklessness. The framed officer is brought before the sith lord for judgement.
The sith, however, is not what people expect. He has a more level head than most, though he can still be cruel. He suspects your character is innocent, and he still sees potential in her. After a bit of consideration, he offers her a choice. She can select death and be executed for her "failure." If she doesn't, she will live a double life. Publicly, she will be stripped of her title and sentenced to a life of serving as the sith's personal girl. She will be expected to obey his every command, humiliating herself in front of those she once humiliated. Behind closed doors, however, she will undergo rigorous physical and emotional training, turning her into the perfect weapon. She will be the sith's hand, secretly eliminating his enemies and protecting his interests, as nobody would suspect his poor slave of being his trusted agent. She will share in his success and have a chance at revenge at those who betrayed her, but the sith expects absolute obedience without thought or hesitation. Failure will not be tolerated.