The Werewolf and Vampire Mastery Achievements
Before we get started on the quests and the adventure in the Dawnguard expansion we need to deal with one significant issue that can cause problems for gamers who prefer not to create alternate saves, and who are purists in the sense that they want to experience one linear adventure using their main character -- the two beast Achievements that are ruled by the fork in the story.
Veteran players will already be familiar with the faction system, which kicked in at the very start of the game when you escaped your own execution -- a system that is used in the Dawnguard story to great effect by basically forcing you to choose the good or the bad side... Either side with the Dawnguard and fight the vampires and their insidious plan, or join with them as a vampire yourself and battle the Dawnguard to extinction -- the choice is yours... But not if you want to obtain all of the Achievements, because then it becomes a very complicated choice indeed.
Even if you are not a purist in this sense, there are two Achievements that may give you trouble -- the two for mastering the perks for the werewolf and the vampire -- which are called "Vampire Mastered" (20 GP) and "Werewolf Mastered" (20 GP), each of which are unlocked by obtaining all of the 11 Perks that are part of the ability trees for each beast form.
The rub is that you gain the ability levels that are required to unlock each perk by killing and then feeding with the appropriate skill. You cannot be a vampire AND a werewolf at the same time, you have to choose one or the other, and you are forced to make that choice only a short way into the story.
Unlocking 11 Werewolf Perks
If you choose to align with the vampire faction you are turned into a vampire, and your story plays out one way, and if you choose to oppose the vampire faction you are not, and you join the Dawnguard in their struggle to slay the vampires -- either as a human or as a werewolf.
So basically there are a few strategies you can take in dealing with this:
(1) You can make a save just before you are forced to decide which faction you will align with, and then play out one to complete its missions and then revert to the save and play out the other;
(2) Assuming you are already a werewolf, you can make a save, then rotate among the cities at night triggering your werewolf form and killing pretty much everything in sight to obtain all 11 perks and thus the Achievement, and then revert back and play through the game aligned with the vampire;
(3) You can play through your choice of alignments and to hell with the Achievements!
One thing you need to bear in mind if you decide to play as a vampire and then have Aela change you back to werewolf in order to get out from under the vampire curse is that she will only do that change ONCE. So use it wisely.
-- The Werewolf --
Basically you change to Beast Form and kill humans -- if you have advanced the perks on the right-hand path you can actually kill animals as well so that should be a priority for you -- and then you feed (using the 'A' button) on the people/beasts you have killed. After you feed a certain number of times -- when you are feeding basically you are eating their hearts -- you gain a perk point.
To speed this up, once you finish killing all the people in the area revert back to human form and then cast Soul Trap on each body to gain another heart. Note that you MUST wait for the message to go away that says you obtained the heart BEFORE casting Soup Trap again.
When you are obtaining the perks with your perk point in order to unlock the Achievement bear in mind that the first choice in the tree requires you to spend four points in it to cap it off.