Skyrim:Bloodline

Quick Walkthrough
# Escort Serana to Castle Volkihar.
# Speak to Lord Harkon.
# Either join Volkihar or remain loyal to the Dawnguard. 

Detailed Walkthrough
Escaping the Crypt
After freeing Serana, exit the cavern by crossing the small stone bridge on the opposite side of the room you came in from, where you will be ambushed by two gargoyles. After dealing with them, head up to the door located to the right of where the gargoyles were hidden and enter a small chamber. This room contains little of note, except for the numerous skeletons and leveled draugr that will attack when you pull the lever to open the next barred gate. Head through the gate to come to a huge amphitheater with several leveled undead pacing about. Here you will find a boss chest with leveled loot and a word wall that teaches a word of the Drain Vitality Shout. Behind the Word Wall is an altar table with a few coin purses and 2 random enchanted weapons.  After dealing with the enemies and getting the desired loot, head out through the gated door at the opposite end of the room from where you came in.

thumb|right|Harkon giving the player character his "gift"

Castle Volkihar
Now, you are ready to take Serana to her father at Castle Volkihar. This location is in the northwest corner of the map. You will be able to see it from a great distance away. The easiest way to get there is to take the boat at Icewater Jetty, which is reachable on foot, by fast travel, or by hiring a ferryman to take you there. Activate the boat and you will arrive at Castle Volkihar.

Bone hawks circle overhead and a stone bridge lined with gargoyles is in front of you. Beyond, the castle looms. About halfway across the bridge, Serana will initiate conversation: she wants you to keep quiet and let her do the talking.

The Watchman will open the gate when he sees his lady. Open the door and step into the castle where Serana's father is waiting. A vampire named Vingalmo will confront you; but he backs off when he sees Serana and goes to announce her to the group gathered in the main hall.

Descend into the hall and watch the reunion between Serana and Lord Harkon. After that, Harkon will initiate conversation with you. He will tell you there is only one thing he can give you to thank you: his blood. You now have a choice to make. You can stick with the Dawnguard and you will be allowed to leave the castle in peace for your service, or you can join the Volkihar vampires and take on the mantle of a Vampire Lord.

Choosing Vampire Lord
If you choose to become a Vampire Lord, Harkon will bite you as everything fades out. When you wake up, you and Harkon will be in the Volkihar Cathedral. Harkon will begin teaching you how to use your Vampire Lord powers, which include raising dead and draining life, as well as a claw attack. He also explains a few things, such as his history and that of his family, if you ask him about them. If you are already a vampire, he will say you just contracted a disease and explain that what he offers you is true vampirism. Alternatively, if you happen to be a werewolf at the time, he will state that he can smell it on you, and can purge you of its filth. Once you are done getting acquainted with your new powers, you will be given a quest to fill The Bloodstone Chalice.

thumb|right|A skirmish between the Dawnguard and the vampires

Choosing Dawnguard
If you decide to stick with the Dawnguard and refuse his offer, Lord Harkon will cast a spell that will teleport you outside the Castle gate. He also "advises" you to never return. With the castle locked behind you, your only remaining option is to return to Fort Dawnguard and report this new discovery to the Dawnguard. Once you arrive, you will find the fortress under attack by a band of vampires. After defeating the vampire attackers, you should speak to Isran and tell him what you discovered. Following this report, he will give you the duty of seeking out some of his old allies to create A New Order. Following the Dawnguard side of the questline will cause Fort Dawnguard to be rebuilt and refurnished. You will also have access to Crossbow upgrades and Armored Trolls.

 

Notes
While in the castle during this quest, you can't draw your weapon, cast spells, shout or do anything aggressive. However, you are free to take anything in the keep without it being identified as thievery, although many of the doors are locked (and can't be picked), and there is very little time to do so – just two gaps in the forced dialogue. 
You can still become a Vampire Lord if you choose to side with the Dawnguard, though it will not be possible until later in Dawnguard's main questline. If you do, your allegiance to the Dawnguard remains unchanged, though many of the Dawnguard will treat you with hostility and not offer services until you're cured.
Siding with the Dawnguard allows access to some unique rewards. Specifically, the Ancient Technology quests allow you to smith crossbows and crossbow bolts, both steel and dwarven, and Lost Relic quests allow you to recover three unique magic items. Similarly, if you join the Volkihar side, the Ancient Power, Amulets of Night Power and Rings of Blood Magic quests help you enhance your Vampire Lord abilities.
If you exit the conversation when Lord Harkon tells you to make your choice, and take too long to give an answer, he will banish you from the castle as if you chose to refuse his gift.
If you are a werewolf, siding with Lord Harkon will cure your lycanthropy. However, you can become a werewolf again by asking Aela (which will cure your vampirism), and any lycanthropy perks which you had will be restored. You can restore your lycanthropy this way only once. You cannot advance in the Volkihar questline or use any of the vampire clan's services except when you are a vampire.
If you try to bring Serana to Fort Dawnguard, she will refuse to go inside it, having "a bad feeling about this place". Isran will tell you to do as she asked in order to discover what it is all about, even if you tell him she's a vampire.

Bugs
 
You can circumvent this issue by clipping through the wall on the right while standing on the railing. This gets you past the gate and able to access the door.
You can glitch through the gate by holding up a large plate/platter against the gate, then using Whirlwind Sprint to sprint into the gate and pass through it. See YouTube for demonstrations of the plate glitch technique.
  Use the command   to move through the wooden gate and enter the castle to continue the quest.
  use console player.moveto 03003ba8 to meet Harkon inside the castle and that would have you start quest next quest. however, you have to fast travel to complete the quest "bloodline".
  use console disable (02017d59) to remove the gate
 
 Target Serana with the console when outside, use   command once inside, then  .
 
 
If you are unable to start the quest after you have turned into a vampire and you try talking to Garan Marethi, all he tells you is to go back to Lord Harkon and get changed back to a vampire, and you are unable to start the conversation with him. To fix this on PC, do the following: open the console, and type   player.setrace  , replacing     with the name of your race (e.g.,   if you are a Dark Elf).
 
 You can fix this by entering either   or  , followed by   or   into the console. 
 If the above does not work playerfollowercounts value may be the issue. To fix this load a save from BEFORE you open Serana's sarcophagus. Dismiss any current followers, using   if necessary (may as well to be safe) then proceed through the quest as normal.
 
 
 
 Start a new game without Dawnguard installed (don't enable Dawnguard, for users of a mod manager), Save, Exit the game, Install Dawnguard (enable Dawnguard again, for mod manager users), Start the savegame previously made without Dawnguard and the bug will not appear in that save. This procedure will not allow you to remove the bug from other savegames made with Dawnguard installed/enabled. (Tested with Nexus Mod Manager for an easy uninstallation/installation of Dawnguard.)
Reload to a save after entering Dimhollow Crypt, wait 30 days, then proceed with the quest. this resets all dungeons, characters, etc... this also fixes the bug where you die after being bitten by Harkon.
 
 Before talking to Harkon, use the console commands   followed by  . Talk to him and he will say that you have forsaken his gift, but will offer it to you again. Simply accept it and you will gain the Vampire Lord power without dying from his bite.  
You can also try standing behind one of the tables in the room before Lord Harkon addresses you.  From there, he won't be able to lunge at you so you won't die, but you'll receive his gift anyway.
 
This bug may be due to using mods that alter followers behavior. If you can't enter the castle the first time with Serana you can use the console to move in, open the console and type  player.moveto 020033b3, prid 02002B74, moveto player  and move back to the door. That way the quest remains untouched.
You can also change to your human form, go into sneak mode, and type   in the console to reach the door to Skyrim undetected. After you exit the castle you may enter again as normal.
Write in the console   and they won't attack you but remember when you go out the castle just turn it off by writing   again in the console.
 
In the latter case, it may be that the follower is actually inside the castle, rather than simply gone.
 
 
 

Quest Stages