"I've met Isran, the leader of a group of vampire hunters known as the Dawnguard. The vampires that attacked the Hall of the Vigilants may have been after something in a place called Dimhollow Crypt. Isran has asked me to find out what the vampires are looking for."
Before we get too far into this you should be aware that the first part of the quest line is the same regardless of what faction you will be joining -- basically you will be completing the quest AWAKENING and then you will start the quest the follows -- BLOODLINE -- at which point you will be required to decide which faction you will be supporting.
The story for each is substantially different, and so if you do want to experience both I recommend that you create a save and be sure not to overwrite or delete it just before you are to make the choice. That way you can do the faction you do NOT want to do first, then revert to that save and do the faction that you want to complete the expansion with, as that way when you complete it, you are good to go for the next expansion when it is released. Fair enough? Naturally!
So with that in mind this guide will cover both faction lines, and at the appropriate point you simply need to jump to the correct one for your game to proceed. And now, with that out of the way, let us get started on the first official quest of the expansion: AWAKENING. Before you head out for this quest, assuming you do NOT wish to be turned into a vampire until you are ready to choose to be, you should bring a supply of Cure Disease potions with you just in case you get bitten/slashed as you can easily get infected that way. When you are ready, head for the marker...
-- Before You Begin Your Journey --
Prior to setting off for Dimhollow Crypt there are a few minor things you should look to, starting with your kit. Obviously you should be wearing your best armor, and have your best weapon with you. You can take the Crossbow that the Dawguard gave you and, at the Smithy, use the grindstone to make it a Legendary Weapon. Then hit the Enchantment Table in the castle to Enchant it -- adding damage multipliers is always a good idea -- for example I added fire and shock to mine -- as that will make it even more effective.
Before leaving home you should dip into your ingredients chest for the following items:
-- Charred Skeever Hide
-- Hawk Feathers
-- Mudcrab Chitin
-- Vampire Dust
Using those, mix a batch of Cure Disease potions... Bearing in mind that while Cure Disease potions will NOT cure Vampirism, they WILL cure Sanguinare Vampiris, which is the disease that leads to Vampirism and that you will catch while fighting vampires. For that reason these are important since if the infection is allowed to remain beyond two in-game days, it becomes full-blown vampirism.
If for some reason you do not have those or want to take the trouble, hit your local potion merchants and buy some Cure Disease potions for the adventure. I would think that 12 or so potions should be enough...
From your Smith supply chest grab some Dragon Bone, Steel Ingots, and Firewood, and then use the Smithy to make a supply of Steel Arrows and Dragon Bone Arrows for your bow -- ideally you would be better off making Bolts, but the ability to do that as well as make better Crossbows is part of the perks you get for siding with the Dawnguard so you will not be able to do that until later.
You should have some of the bolts you got from the Dawnguard still in your inventory as well as more than a few Dwarven Bolts assuming you were looting all along in the ruins we have been adventuring in -- I sure hope that you have!
Check your supply of Soul Gems -- which you will need to recharge the enchantments on your weapons -- and check your weapons now to see if they need to be recharged and do that. If you are low on filled Gems you may want to grab your Soul Sword and some empty gems and go fill them now...
Completing the Brazier Puzzle to Unlock Serana
-- A Journey to Dimhollow Crypt --
Following the instructions of the leader of the Dawnguard we need to head to Dimhollow Crypt -- if you have completed the main game story you should be able to fast travel to Frostmere Crypt or Mzinchaleft (those are the closest spots you should be able to fast travel to), and then head to the marker in order to "discover" the crypt.
When you arrive and enter the Crypt you will be confronted by a Death Hound and some Vampire -- Vigilant Tolan is dead, he lost his life in his battle with the Vampire -- so you need to take them out as quickly and as efficiently as you can manage. There is a gate here but it is closed and there are no controls to open it near it -- exploring the cavern reveals a set of stairs and some loot and -- coolness -- a pull-chain that opens the gate!
At this point you will proceed up into the tower, battling the odd Vampire and undead skeleton or three, and eventually you will battle your way through a tunnel to a fight with a Boss Vampire -- at this point all things being equal you may want to go ahead and drink a Cure Disease potion since the odds are very good that you have been infected... Just saying.
Opening the last gate to the end area you listen to a conversation between the Vampire Lokil and Vigilant Adalvald before Lokil slays Adalvald, at which point you will battle and kill Lokil yourself. Now, before you do anything else go loot the body of Adalvald and read the book he has, Notes on Dimhollow Crypt, Vol. 3, because it will offer you a clue you need to proceed.
Once you read the book you now have a better informed notion of what this place actually is -- and once you have killed the interloping vampires and their Thrall, you can begin unlocking its secrets for yourself!
Start by sacrificing a little of your own blood to activate the tomb, pressing the button on the pedestal that pierces your hand and powers the glowing field. A close examination of the layout should clue you in that you need to move the Braziers in order to light up the lines and trigger the raising of the tomb! Opening it reveals a mysterious woman who turns out to be Serana -- a vampire who wants you to help her get back to her family!
Once you complete the first part of the dialogue options you complete the quest AWAKENING and trigger the quest BLOODLINE. This also unlocks the Achievement "Awakening" (20 GP) for finishing the quest.
There is an abundance of additional dialogue for you to complete with her, though to be honest Serana does not appear to be very forthcoming with actual information, so you will have to glean what you can from the little that she does say...