The below post was taken from the Elder Scrolls Online Guide from KillerGuides with approval. Several more detailed chapters for the nightblade are waiting for you to discover. It's by far the most detailed guide I have read when it comes to how to level up your nightblade.
In The Elder Scrolls Online, players will be immersed in Tamriel and choosing three Alliances where you must pledge your allegiance to. Which Alliance will you choose to bring forth honor and glory for your monarchy and ultimately, faction supremacy?