Vampire Faction Chosen
Well you have made your choice, and your choice is to be a nearly omnipotent blood-sucking fiend who had better fear the blonde cheerleader from Sunnyvale High, I am just saying... Actually in the world of vampire lore I have always preferred the movie My Best Friend is a Vampire over Buffy the Vampire Slayer, because I like the warm fuzzy vampire not the sinister violent death-dealing type, so of course I will be slightly disappointed as it is the latter and not the former we have here!
After accepting the "gift" Harkon wastes no time and lunges at you, grabbing you and sinking his fangs into your neck, and the transformation commences as you black out! When you come to in a wobbly I-have-just-had-the-most-intense-pub-crawl-of-my-life morning after, you find that there is a bit of learning to go with this gift, but fortunately for you, Harkon feels in the mood to teach you!
You can ask him what happened and how you got there and learn the story behind how he became a Vampire Lord and the relationship that one of the Gods had in the process -- and then you can tell him you are ready to be trained -- after which he does just that!
First he directs you to assume your Vampire form -- I suggest that you go ahead and mark that as a Favorite now so you can use the D-Pad Menu later to make it easier to select -- and you may want to mark as Favorite some of the Vampire Abilities that are also available to you, though to be honest the one that you will likely use the most is Vampire Servant... Anyway once you have done that go ahead and assume the form.
Becoming a Master Vampire Lord
Harkon then tells you about your powers -- right hand is drain, left hand is raise a dead body as your servant -- and then he asks you to land on the ground (push down on the left joystick) and he explains why being ground-bound makes you more vulnerable and your powers weaker.
Your next lesson is how to feed -- and what that process does for you -- you can ask him about the things you should know about being a Vampire and you should really ask these questions. After all you would not buy a car without asking questions about it, so why would you suffer a life-altering event and not ask questions?
Once you are happy with the lessons and chat, go ahead and tell him you are ready to accept a mission from him now, and he will direct you to deliver a message for him. This completes the quest BLOODLINES and starts the quest THE BLOODSTONE CHALICE.
On your way out of the chamber you will notice a human "Thrall" -- a human that the vampire use as feeding cattle -- who you can feed upon by reverting to human form, moving behind her, and then going into Sneak-Mode and selecting her, choosing Feed in place of Pickpocket -- and voila! You feed!