Skyrim:Chasing Echoes

Quick Walkthrough
# Speak to Serana.
# Explore Castle Volkihar.
# Investigate the Moondial.
# Explore Castle Volkihar's Ruined Tower.
# Locate Valerica's Journal.
# (If not a vampire) Become a Vampire Lord or get partially soul trapped.
# Enter the Soul Cairn.

Detailed Walkthrough

Serana will want to speak to you after the Moth Priest reads the Elder Scroll, saying she may know where to find one of the scrolls. She thinks her mother, Valerica, may have information on it, and possibly may even possess it. After going through her dialogue, Serana believes Valerica may have left a clue at Castle Volkihar, in the courtyard. You will not be able to enter through the Keep, but there is a secret entrance used by the previous owner to obtain shipments. Note that if you have a follower at this point, you must dismiss them. Simply telling them to wait is not enough. Serana won't accompany you unless you send your follower home, and you'll need her later on.

When you reach the castle, head left, along the coastline. Serana will mention you're getting close to the entrance. When you reach the castle's old docks, you will be attacked by skeletons (more powerful than average skeletons). Once you enter through the secret entrance to the castle, you will have to fight your way through death hounds, skeletons, and a feral vampire. After dealing with the feral vampire, you will arrive at a stone archway blocked off in the western corner of the room. Ignore this for the time being and proceed instead up a set of stairs on the opposite corner of the room, which will lead you to a lever needed to lower the bridge blocking passage through the stone archway. Once on the bridge, take a left as instructed by Serana. This leads into an open area inhabited by death hounds. Afterwards move up the stairs in the southern section of the room which takes you into a tunnel. Eventually you must fight a giant frostbite spider and pull an additional lever to lower a bridge, allowing access to the courtyard. Just before you hack through the web to reveal the giant frostbite spider, there is an adept-locked chest to your right hidden behind more spiderwebs. If you don't go right into the room ahead, the giant spider can be killed with long-range attacks as it's too big to come down the small tunnel. When you reach the courtyard Serana will tell you there is something strange about the moon dial, and you have to find all three missing pieces and place them in the moon dial for a secret entrance to open.

The Half Moon Crest can be found in the center of a pond lying against a rock in the northwest corner of the courtyard.
The Full Moon Crest can be found behind a patch of nightshade lying against the castle's exterior wall in the fenced off area identified as Valerica's garden.
The Crescent Moon Crest can be found up the stairs just to the right of the entrance to Volkihar North Tower lying on the floor beside a table and chairs.
thumb|right|Serana opening the portal to the Soul Cairn
Afterwards, you will have to continue fighting gargoyles and skeletons until you reach a room with gargoyles and a fireplace. Serana will tell you there must be a secret entrance somewhere. Turn the candlestick near the fireplace or pull the chain behind the gargoyle to reveal the door (there is a vampire royal armor near the shelf to the right). Inside the laboratory you will need to search for Valerica's journal which is located on the central bookcase to the right of where you entered. After Serana reads it, she will tell you to find certain ingredients which can be found in the laboratory:

The Finely Ground Bone Meal is on the table by the entrance beneath the mammoth skull.
The Soul Gem Shards are on the first wardrobe up the stairs that head to the left.
The Purified Void Salts are on a shelf with other ingredients, near the farthest point of the raised area of the room, on the balcony above the entrance.

All of these items are in silver bowls, which will not be added to your inventory when you pick them up. These are the items you must have; normal bonemeal, soul gem fragments, and void salts will not work.

Find them and place them in the vessel in front of the portal (Serana will typically wait for you beside the vessel, making it easier to locate). Serana will then add her blood to the mixture and the Soul Cairn portal will start to open. If you are not a vampire and you attempt to enter the portal, it will leech some health, so return to Serana.

Serana will tell you to enter the Soul Cairn you must be "dead". She will then give you a choice:

Allow her to turn you into a Vampire Lord.
Allow her to trap part of your soul in a soul gem, which will add the Weakened Soul effect, causing you to have 45 points less of health, stamina, and magicka and reducing their regeneration rates while in the Soul Cairn. This is the only way to enter the Soul Cairn without becoming a vampire. The effect is removed once you find the Soul Essence Gem.

Choosing either will allow you to enter the Soul Cairn.

Notes
While this quest or the quest Beyond Death is active, you will be unable to dismiss Serana from your party. Though the option to part ways with her is present, no matter how many times you click on it she will remain a quest follower until this quest and the following quest are over. If you are on the Dawnguard side and are a werewolf, this can be extremely frustrating if you use Beast Form and have the Savage Feeding perk as Serana tends to raise corpses in battle, thus disallowing any feeding to extend bloodlust. For this reason, it is strongly advised not to use Beast Form at all.
There is a small window of opportunity where you can feed on raised corpses after they die again but before they turn into a pile of ash. One tactic, therefore, is to kill any enemy Serana raises yourself before it dies due to the spell's effect ending, and to quickly try to feed on it before it turns to ash. 
It is not necessary to be a Vampire Lord to enter the Soul Cairn without needing to be soul trapped. Being a regular vampire works as well. If you side with the Dawnguard, you may choose to become a normal vampire for the duration of this and the next quest, then cure yourself after leaving the Soul Cairn. However, if you've entered the Soul Cairn for this quest as a vampire, you will have to be a vampire on subsequent trips.
From the courtyard you have access by door to two more zones, the north and east towers. The east tower is empty with some minor loot (gems and coin), the north tower has a few skeletons and a gargoyle plus also some loot.
Once the quest ends and you emerge from the portal, the fastest way to leave the castle is to exit through the door that leads to the "Castle Volkihar Balcony". This is an outside area that can be fast-traveled to and provides easy access to the Soul Cairn without having to traverse the castle ruins and the courtyard again.
If you use this door before entering the Soul Cairn, Serana will stay in the study even if you fast travel, making an exception to the above rule about dismissing from party.
There is a chest convenient to the door so that one can load up to maximum weight minus one, fast travel to merchants and repeat, as well as visit trainers, do other quests, etc.
There is a small window of opportunity to acquire two followers here, Serana and one other. From the point where Serana is waiting for you to fill the vessel with the special ingredients, until you decide whether you want her to turn you into a Vampire Lord or capture your soul, you can leave via the balcony door. You now have the opportunity to leave the castle grounds and recruit another follower. Once you do and fast travel back to the castle balcony, Serana will automatically rejoin with you and your follower will not be dismissed. Your follower will not enter the Castle (neither will a dog companion). However, when you finish with the Soul Cairn and leave via the balcony again, your recruit will still be there waiting. Serana will continue traveling with you. If you take advantage of this one opportunity before completing "Stage 100" below, you can have two followers for the rest of the game. You will still be able to change the 'normal' follower, dismiss him or her and pick another instead, continuing to have two followers in the group, but you can't dismiss Serana. If you dismiss her this opportunity will be lost.
If for some reason you you to lose the second slot upon entering the Ancestor Glade for that quest there will be a second opportunity at the end of 'Touching the Sky' immediately after you kill the boss for that quest to fast travel elsewhere and return with a second follower - at which point you'll have a second slot for the rest of the game, although it's recommended not to dismiss them until you've killed Harkon at the end of Dawnguard and returned to normal game areas. Dismissing Serana at any time will always eliminate your second spot so don't do it, ever.
Choosing to become a Vampire Lord and then returning to Fort Dawnguard will prompt Florentius, Sorine, or Gunmar to direct you to Falion, thus beginning the quest Rising at Dawn. Other Dawnguard members, including Isran, will regard you with caution and disgust. You will not be able to use the Dawnguard's services until you cure yourself.

Bugs
 
 It is possible to manually advance to the next stage of the quest by opening the console and typing   .
 This can also be caused by multi-follower mods. Typing 'set playerfollowercount to 0' will let the quest continue normally. Once back out of the Soul Cairn set the value back to the number of followers you have.
 
 It is possible to manually advance to the next stage of the quest by opening the console and typing  .
 
 
It can be dropped or stored once you rescue her mother and return from the Soul Cairn.
 
 Open the console and type  .

Quest Stages