Speak to Serana to get things started. When she asks where her mother might be, being wrong in your initial choice has no real effect besides some conversational detours.
Head outside and around to the alternate entrance on the left side of the castle. From here, the quest is straightforward until you reach the courtyard. You’ll have to find all of the missing crests and place them in their appropriate spots on the sundial. The three missing pieces are in the immediate vicinity of the courtyard and are easy to spot (they glimmer).
Adding the pieces opens up a secret path leading underneath the courtyard. (If you’re curious about the towers surrounding the courtyard, they are fairly small rooms containing some skeleton enemies and some valuable jewels, ingots, and jewelry.)
Continue on until eventually you find yourself in a room full of gargoyles. Once you’ve defeated them, Serana will mention something about a secret passage. Go over to the fireplace and turn the unlit candlestick on the left.
Once inside Valerica’s Study, talk to Serana. You’ll then have to find her mother’s journal. The small red book is on the top shelf of a bookshelf in the southern corner of the room. Show it to Serana, and you’ll begin a quest to gather some materials. The materials are outlined in your quest notes and are all in the room with you.
Once you’ve put the materials in the vessel, the portal to the Soul Cairn will open.
If you are a Dawnguard member, you’ll have to make a choice before entering the Soul Cairn. As Serana will explain, the area is hungry for life. A normal human cannot survive inside. If you allow Serana to turn you into a Vampire Lord, you can enter without being harmed. Just make sure to complete the “Rising at Dawn” quest (the standard quest to cure vampirism, started by speaking to Falion in Morthal) before returning to Fort Dawnguard.
The non-vampiric alternative is to allow Serana to perform a partial soul trap ritual on you. This will weaken you inside the Soul Cairn, but you will remain human and alive.
Enter the Soul Cairn and reach the waypoint with Serana in tow and she’ll begin talking with her mother. Your new goal is to kill the three Boneyard Keepers as a means to lower the barrier and gain access to Valerica. All three Keepers are marked on your map, so simply follow the arrows.
Once you’ve defeated the keepers, return to Valerica, speak to her, then follow her into the boneyard. Here is where you’ll have to take on Durnehviir the dragon. It’s not a particularly difficult fight, but the bonemen that spawn can complicate things. Valerica and Serana will handle most of the bonemen, but because of the sheer number that spawn, you’ll still have a handful on your tail as you try and deal with Durnehviir.
After the battle, grab the scroll and speak to Valerica. It is now time to leave this place, but if you want to take on some of the side quests before you do (though you can always return to the Soul Cairn), move onto the side quest section.
As you leave the boneyard, Durnehviir will teach you the “Curse, Summon Durnehviir” shout, which summons Durnehviir into battle to aid you.
The Dragon Elder Scroll
The second scroll you need is the Dragon Elder Scroll. If you have already completed the quest “Discerning the Transmundane” from the vanilla Skyrim quest bank, you will already have the Elder Scroll in your possession. If you happened to sell it to Urag gro-Shub, go to the College and buy it back.
To begin “Discerning the Transmundane,” seek out Septimus Signus in his outpost north of the College of Winterhold (pictured above). The quest is lengthy, but at the end, the scroll will be yours.
Now that you have both scrolls, you can complete this quest. Return to Volkihar and speak to Dexion. He’ll give you your next quest.
You’ll have to travel all the way to Ancestor Glade, which is southeast of Falkreath. Once inside, grab the knife, then gather bark from one of the surrounding trees. Once you have it, run around the area looking for moth swarms. Once you’ve gathered seven, stand in the light, select the Blood Elder Scroll, and read it.
Touching the Sky
Your next destination is Darkfall Cave, just south of Mor Khazgur. Move through it until you find Gelebor, who will give you a complicated task before helping you get Auriel’s Bow. Continue forward through the portal into the Darkfall Passage.
You’ll have to move through the passage until you hit a wayshrine. Draw water from the basin and pass through the portal behind it to arrive in the Forgotten Vale. Here is where you’ll find three more basins (marked on your map) to draw water from.
The final basin is in the Glacial Crevice. To reach this area, head towards the marker on your map. (You’ll notice that it indicates the final basin is in an area beyond the Forgotten Vale.) Work your way upriver to reach it.
Pass through the Glacial Crevice and you’ll emerge onto the Forgotten Gale - this time closer to the last wayshrine. Make your way through the Falmer village. Once you’ve reached the final wayshrine and filled your basin, turn around and enter the giant temple in the distance.
Pour your water into the basin near the main entrance to gain access to the temple. Once inside, you’ll come across several doors that require you to place your ewer on a nearby altar to open the path. Once you do so, pick up the ewer (so that you can use it again) and make way through the doorway before it closes. There is always another altar on the other side, so don’t worry about becoming trapped.
Proceed until you reach Vyrthur. After speaking with him, all of the frozen beings in the room will reanimate and attack you. Defeat all enemies, then follow Serana up to the balcony. Here you’ll have to face and defeat Vyrthur. Finally, you will be allowed to take Auriel’s Bow and complete this quest.
Speak to Serana. Instead of taking the portal back to Darkfall Cave, simply fast travel out of the area (you are outside at this point) and to Castle Volkihar. Once inside, enter the cathedral and speak to Harkon. You can wait for the lengthy dialog, or just attack - either way, a battle will ensue.
Harkon is a difficult target because he will slowly regenerate his health throughout battle and uses his bat form to teleport around the room. If you have some means of stunning him (such as high-level dualcasting), by all means use it.
He’ll also summon skeletons and gargoyles to fight alongside him. You may want to avoid spending time dealing with these and instead chase Harkon around the room. The longer you are distracted, the more he will heal.
Once he takes a certain amount of damage, he will return to the center of the room and surround himself in a red healing aura. At this point, whip out the Auriel Bow and fire it as quickly as you can so as to stop his regeneration.
Notable loot from Harkon’s corpse includes Vampire Royal Armor, which is great for mages as it not only offers the ‘magicka regenerates 125% faster’ perk, but it provides defense, unlike robes. You’ll also get Harkon’s katana, which, while fairly weak, absorbs 15 points of health, magicka, and stamina if wielded by a vampire. Serana can also become a permanent follower at this point if you wish. She hangs out at the castle otherwise.
If you are a Vampire Lord and have some elven arrows, you can now ask Serana to coat them with her blood. If you use Auriel’s Bow to shoot these arrows at the sun, you can turn it red, nullifying the negative effects it has on you as a vampire (for the rest of that day only).