Discerning the Transmundane

It serves as the Daedric quest for the prince Hermaeus Mora. It also runs concurrently with the main quest "Elder Knowledge," and is the only other method of accessing Blackreach other than Elder Knowledge.

Background
"I met Septimus Signus, a brilliant but mad scholar living in the northern ice fields. He's directed me to a Dwarven observatory that supposedly houses an Elder Scroll. He doesn't care about the Scroll itself, but wants me to use some kind of machine there to inscribe a Dwarven lexicon with the Scroll's knowledge and bring it to him."

Walkthrough
Although this quest can be started at any level, such as during the main quest "Elder Knowledge," the second part involving the harvesting of blood can only be initiated after level 15.

It can be started by speaking to Septimus Signus, a brilliant but mad scholar who has devoted his life to studying the Elder Scrolls. He can be found in Septimus Signus's Outpost, an ice cave north of Winterhold. He explains that the Dwemer lockbox buried within the outpost holds the "heart of a god" and he wishes to access it to learn its secrets.

He gives the Dragonborn an Attunement Sphere and a Blank Lexicon which must be inscribed with the Dwemer mechanism located within the Tower of Mzark. The tower can only be accessed via Blackreach, the underground ruins of an ancient Dwemer city.

Alftand Glacial Ruins
The first step is to travel to the Dwemer ruin of Alftand, located in the snowy mountains southwest of Winterhold. Enter the Alftand Glacial Ruins and progress through the tunnels of ice. Eventually a Khajiit skooma addict, named J'darr, will attack. His brother's journal reveals that he, along with several others, were members of a failed expedition to the ruins. Proceed through the caves and past two Dwarven spiders until the Alftand Animonculory is reached.

Alftand Animonculory
Proceed through the ruins and fight past the various Falmer that reside here. Several dead members of the failed expedition can be found here. Eventually the Alftand Cathedral will be reached which grants access to Blackreach.

The Alftand Cathedral
Head past the traps and Falmer who guard this section. Past a set of double doors opens into a large room patrolled by several enemies. Head through the main gate where two Dwarven centurions can be found. One Centurion lies on the floor inactive, however the other will awaken when approached. Defeat the Centurion and take the key from its corpse. Proceed to the next gate where a Redguard named Umana and an Imperial named Sulla Trebatius begin to fight, after a brief exchange of words. If one is killed, the other will turn and attack the Dragonborn.

Proceed towards the Dwarven mechanism located in the center of the room. Activate the mechanism with the Attunement Sphere which causes the floor to retract into stairs. Head down the stairs and into Blackreach.

Blackreach
Progress through the vast cavern towards the objective marker, fighting off the various creatures within. Once the Tower of Mzark is reached, near the southwest corner of Blackreach, proceed inside and through the corridors to the main room. Head up the spiral stairs until the Dwemer mechanism is reached. Place the Blank Lexicon inside the receptacle and begin the inscription puzzle.

Lexicon puzzle
Solving the puzzle involves pushing four buttons on the podium in a certain order. After the Lexicon is placed on the designated receptacle, the two buttons on the right side will light up.

To decipher the code, press the third button from the left a few times until the Lexicon opens and button two lights up. Press the second button from the left twice (until the overhead lenses are seen directing light onto the sphere), then the first button lights up. Press this button to reveal the scroll.

Pick up the Elder Scroll and the runed lexicon from the receptacle. Exit the Tower through the door beneath the button console and return to Septimus.

Return to Septimus
The next part of this quest is dependent on character level. Returning to Septimus at under level 15 will cause him to dismiss the Dragonborn until he has had time to study the Lexicon. When level 15 is reached, he will send a courier to summon the Dragonborn back to the outpost.

Returning to Septimus at level 15 or higher will prompt him to begin immediately reading the Lexicon. He then reveals that he is in the service of Hermaeus Mora, the Daedric Prince of Knowledge.

After studying the Runed Lexicon, he explains that Dwemer blood is needed to open the Dwemer lockbox. Since the Dwemer are extinct, blood from the various Mer races must be collected to create a facsimile. Specifically, the blood of Altmer, Bosmer, Dunmer, Falmer and Orsimer. Septimus then gives the Dragonborn an Essence Extractor and requests that the blood be collected for him.

Hermaeus Mora's intervention
Upon trying to leave the iceberg, Hermaeus Mora confronts the Dragonborn and reveals that as soon as the lockbox is opened, Septimus' usefulness will be over. He then offers the Dragonborn Septimus' position. Responding either way has no discernible consequences.

Harvesting blood
To harvest the blood, loot the corpse of a deceased person with the Essence Extractor in the inventory. Blood samples can be acquired from any dead body in Skyrim.

Liar's Retreat is one of the few locations where all blood types, except Altmer, can be found. If this location is used, then the quickest way to obtain the missing Altmer blood is from the corpse of the Altmer inside the spiked pit trap in Halted Stream Camp. The pit trap can be accessed without entering the mine itself, though it is advisable to go through the mine as falling on the spikes from above can kill the Dragonborn instantly.

Locations containing most blood types

 * Liar's Retreat – contains all blood types except Altmer.
 * Nightcaller Temple – Altmer, Dunmer and Orsimer.
 * Rannveig's Fast – Bosmer, Dunmer and Orsimer.
 * Silverdrift Lair – Bosmer, Dunmer and Orsimer.

Other locations
The following is a list of locations that contain corpses or enemies that can be freely slain and used to harvest blood.

Opening the lockbox
Once the blood has been harvested, return to Septimus. He enthusiastically asks for the extractor and presses it into his abdomen, injecting the blood into his body.

Next he approaches the lockbox and the locking mechanism begins to spin. A tunnel-like entrance appears and Septimus runs inside, but much to his dismay he does not find the answers he was looking for.

Rather than the "heart of a god," all he finds is a pedestal on which the Oghma Infinium sits. Amidst the confusion he begins to levitate and Hermaeus Mora disintegrates him into a pile of ash.

Approach the pedestal and take the Oghma Infinium. Upon leaving the chamber, the Wretched Abyss appears again as a manifestation of Hermaeus Mora. He greets the Dragonborn as his Champion and an option is presented to agree or disagree with his assertion. Again, either answer has no consequences.

After the conversation is over the Wretched Abyss dissipates and the quest is completed.

Trivia

 * If is installed, the Wretched Abyss is replaced by a mass of floating tentacles, which is how Hermaeus Mora appears throughout the add-on.
 * Arch-Curate Vyrthur from Dawnguard cannot be harvested for Falmer blood despite being the same race. This is likely due to him also being a Vampire.
 * Even if the Dragonborn is of an elven race, they cannot harvest blood from themselves to fill the Essence Extractor.

Bugs

 * When Septimus finds the book, Hermaeus Mora might not kill him and the quest becomes stuck. This can happen if staying close to Septimus, trying to talk to him or reading the book before he finishes speaking.
 * Killing Septimus resumes the quest as usual.
 * After Septimus is killed and the book is not read, the Lexicon may lock again with no way to unlock it.
 * If was installed and then uninstalled before meeting the Wretched Abyss, it might be impossible to interact with it, thus blocking the only exit and the entire game. Fix: Re-installing Dragonborn and loading a save from before the encounter, and the dialogue will proceed with the 'new' version of the Wretched Abyss.
 * If the save has the and/or Dragonborn DLCs downloaded and, during the current save, they are removeed, there is a chance that Hermaeus Mora cannot be spoken to, thus trapping the Dragonborn. If attempting to get Oblivion Walker and this is the last artifact, a way to circumvent this is as follows: Before speaking to Septimus after getting the runed lexicon, save the game.Attack Septimus and have him follow the Dragonborn to the entrance of his outpost, then sheath any weapons. Talk to him, then obtain all of the necessary Elven blood. Return to Septimus' outpost. While at the entrance's highest peak but closest to Hermaeus Mora as possible, Whirlwind Sprint over him. Once the take Oghma Infinium has been taken, one will receive the Oblivion Walker Achievement/Trophy. Unless one intends to attempt the platter exploit, reload the initial save of these steps or another save, as Hermaeus Mora will block the only exit from the Dwemer Lockbox. Note: Even if one escapes by using the platter exploit, not speaking with Hermaeus Mora may later glitch the game.
 * After completing the quest, the lexicon and essence extractor are still considered quest items and are unable to be removed from the inventory.
 * After giving Septimus the essence extractor, he and any followers present freeze up.
 * Attacking Septimus then sheathing the weapon and speaking to him may resolve the issue.