I don't know who you have to talk to in order to start the quest, but I do know that it isn't available until you hit 15, and you won't be able to hit 20 until it is done. At least that's how it was in beta.
Wish I could tell you more, but the best I can do is give you this level range to aim for. The most important part is that if you hit 20(chosen your subclass) you can no longer defect. This is to make sure that there is no such thing as a "good" shadowknight, vice versa, etc. blah-blah-blah.