This quest starts when you are looking for an Elder Scroll (even independently of the main quest). For example, you can ask around in the very extensive library of the College of Winterhold. The person in charge of the Arcanaeum there is Urag gro-Shub, who is always looking for valuable books and has corresponding tasks to offer. If you ask him about an Elder Scroll, he initially does not believe that you know what you are talking about. These scrolls are instruments of immense knowledge and possess terrible power. If you try to read them, you will likely go mad and lose your sight. You can learn more about the topic from Urag.

If you ask him directly whether he possesses an Elder Scroll, he laughs at first. He says he does not have one. And if he did, it would be under lock and key. You can tell him that you are the Dragonborn; he will then be willing to provide information on the matter and gives the player two books:

1. "Effects of the Elder Scrolls"
2. "Ruminations on the Elder Scrolls"

Ask Urag about the insane book
If you bring up the book, Urag says it was written by Septimus Signus, the expert on the Elder Scrolls. He used to work here at the College, but he has been gone for a long time. He found a Dwemer artifact and moved north. If you want to speak with him, you will have to search for him somewhere in the ice fields.

Find Septimus Signus ★
You must head into the Sea of Ghosts north of Winterhold and look for anything unusual on your way across the ice floes. The entrance to Septimus Signus's Outpost is located inside an iceberg next to a boat. The iceberg is hollowed out inside and houses a massive Dwemer lockbox. From Septimus's chaotic explanations, you learn that he worships the Daedric Prince of the unknown - Hermaeus Mora. The Prince noticed Septimus's thirst for knowledge and revealed several secrets to him in exchange for a few infamous services. Then he brought him here and disclosed that the Heart of Lorkhan lies inside the lockbox. Septimus is obsessed with touching the all-encompassing spirit of this divinity.

His lord and master trickily locked the heart inside this Dwemer box but does not reveal how to open it. Septimus believes an Elder Scroll will help overcome the locking mechanisms. For not even the most sophisticated devices of the Dwemer can withstand the all-encompassing sight of an Elder Scroll.

Septimus also knows where such a scroll is located. The problem, however, is that you cannot read it without taking damage. But the Dwemer built a special apparatus for this, which is said to be located in the Tower of Mzark. To get there, you need the Attunement Sphere - a special key to enter Blackreach. Septimus is not interested in the scroll itself - the player can keep it. He only wants you to read the contents of the scroll using the Dwemer instruments there, store it inside a Lexicon cube, and bring it back here.

Transcribe the Lexicon
Septimus talks a bit more in his chaotic language and partly in verse, explaining that Blackreach can be reached through the ruins of Alftand. Then he explains how and why it is necessary to copy the knowledge into the cube. The more you question him, the more confusing his answers become.

The ruins of Alftand are best reached by marching west from Winterhold. Next to the elevator tower (which is locked at first), you will come across a weathered expedition campsite. Following the wooden catwalk leads you into the Alftand Ice Ruins. Many scattered pieces of equipment can be found in the icy tunnels, along with frequent bloody signs of combat. You will reach a workbench where metallic spider constructs were being examined. The gate to the north can be opened later from the other side. As you progress, you will encounter the members of the expedition, mostly dead or driven mad, like J'darr, who attacks immediately. The ice tunnels transition into stone corridors and chambers of Dwemer architecture, and you must expect attacks from Dwarven Spiders, Dwarven Spider Workers, and Dwarven Spheres The entire way down is also secured by traps: flame spouts, piston traps, spinning blades, and Falmer claw traps. Here and there, you will also find Dwarven chests with novice, adept or expert locks. Some valuable loot can be found behind pickable doors, and small puzzles also lead to treasure chests.

The path leads through an oil-stained chamber into a forge, then through further corridors to the Alftand Animunculatory. There, you will naturally have to deal with attacking Dwarven automatons, and a deadly trap also awaits the careless. Afterward, you must descend into a deep chasm. In the middle of it, the winding walkway has collapsed, and you have to jump. The deeper you descend, the greater the danger of encountering the Falmer who live here. Unfortunately, they are all hostile, and it is almost impossible to sneak past everyone. They live in their dwellings and defend their territory very aggressively. In the second to last room, you should open a gate to an elevator in the northwest. You should take this elevator up once to unlock the gate there (in case you want to shorten the trip during your next visit).

Once back down, you enter the Cathedral area. You should look around here and eliminate all Falmer who could interfere in the upcoming fight. Then, pull the lever above the entrance you just came through to retract the spears blocking the way. On the middle platform, a fearsome Dwarven Centurion will activate and attack. It is advantageous to defeat it because it carries a key. After also fighting two thieves, you can use the key to enter the elevator at the end of the Cathedral, which takes you up to the Alftand Glacial Ruins where you initially found the expedition campsite. You should open the door up there with a lever so that you can reach the Cathedral directly during your next visit.

With Septimus's Attunement Sphere, you can activate the Dwemer mechanism, which creates a staircase in the floor leading to the entrance of the underground area of Blackreach.

You enter a colossal cavern, a gigantic air bubble with lakes and rivers, streets and buildings, hills, valleys, and groves of giant mushrooms. There is a great deal to discover here. To reach your actual destination, take the paved path to the left, then downhill to the southwest, past a Dwarven Centurion and entrenched Falmer archers. The path continues south to the river. Then turn right to the west across the bridge, past the southern main gate of the city, and follow the path along the river to another bridge that leads to the Tower of Mzark.

In the main room, there is a massive metallic sphere with several lenses arranged above it. This Oculory is used to safely read an Elder Scroll and must be operated correctly at the control console. You can reach the control switches via a ramp. First, you must place Septimus's Lexicon cube into the receptacle provided on the far right. Then, you must control the lenses and reading instruments using the switches so that the Elder Scroll is finally revealed, its contents are read, and then transferred into the cube.

How to control the reading process?

The switches activate their respective neighbor to the left. Initially, the switch on the far right has already activated the second switch from the right – this is the state when you enter the room. Therefore, you must press the second switch from the right 4 times until the second switch from the left becomes activated. You should wait for the triggered movements to finish each time. Once the second switch from the left is active, you must press it several times as well, until the leftmost switch lights up. This switch starts the final sequence of the reading process and deactivates all switches again.

Give the Lexicon to Septimus
If you have operated the controls correctly, you can now take the Elder Scroll from its holder ★. Do not forget to take the transcribed Lexicon cube, then head into a side room and take the elevator to the surface. The goal is to deliver the cube to Septimus Signus, who is eagerly waiting for it at his outpost.

However, when Septimus examines the inscribed Lexicon cube, he realizes that another security measure prevents its use. Apparently, Dwemer blood is required to resolve the tiny fractals on the cube. Since the Dwemer no longer exist, Septimus wants to trick the mechanism using a blood mixture from all remaining Elven races. You are handed an extractor to collect the five different types of Elven blood.

Harvest High Elf blood / Wood Elf blood / Dark Elf blood / Falmer blood / Orc blood ★
No clues are provided as to where to find suitable "victims". This is left to the player's imagination.
Upon leaving the outpost, you encounter a dark purple magical vortex blocking the exit. This Wretched Abyss introduces himself as Hermaeus Mora, Guardian of the Unseen. He states that he has watched the player and is impressed by how they master their path, which unfortunately makes Septimus increasingly redundant. Once the Dwemer lockbox is opened, you are expected to take Septimus's place as his emissary. Whatever response you give to pass has no impact on the further course of the game.

Bring blood to Septimus
Once you have collected all the blood samples and delivered the extractor to Septimus, he prepares the blood mixture, opens the lockbox, and runs inside impatiently. However, when he reaches the pedestal inside and is surprised to find no heart, his master "dismisses" him, leaving only a pile of ash behind.

Retrieve the Oghma Infinium
The book on the pedestal is an ancient artifact containing the knowledge of the ages, as it was once revealed to Xarxes. It appears every few years or centuries in the hands of a hero, only to disappear again. You should take the Oghma Infinium from the pedestal and speak with the Wretched Abyss once more. You learn that Septimus was merely a tool to bring the book to light. However, Hermaeus Mora now places it into the player's hands and wishes to accomplish wonders together with them.
When the Wretched Abyss vanishes and clears the exit, the quest ends.

Tips & Notes

• If you ask Urag gro-Shub about rare books, he assigns side quests such as "Shalidor's Insights".
• Right opposite the entrance to Blackreach lies Sinderion's Field Laboratory, along with Sinderion and his journal. As soon as it is read, a typical Sinderion quest starts: "Return to Your Roots".
• There are more than 30 Crimson Nirnroot plants in Blackreach. Once collected, the quest can be completed at another location, where more about Sinderion's past can be learned.
• Blackreach has also been taken over by Falmer, who attack on sight. Other enemies include Dwarven automatons, Chauri, a giant, spiders, wisps, a wispmother, and perhaps a frost troll.
• Southeast of the entrance to Blackreach, an elevator leads to the surface near the "Nightgate Inn".
• In the Oculory room lie the remains of Drokt, who failed at the puzzle.
• Septimus is not interested in the Elder Scroll. It can be kept for the main quest or sold to Urag gro-Shub for 2,000 gold.
• When returning to Alftand with the extractor, Falmer blood can be harvested immediately. The corpse of Endrast (Wood Elf) can be found in the Animunculatory, and that of Yag gra-Gortwog (Orc) on the winding descent afterward. Further down lies the corpse of Valie (High Elf).
• There is no time pressure when collecting the blood samples. The extractor can be used whenever the opportunity arises during other quests.
• The last words of Septimus are: "What is that... it's... just a book?! I can see. The transmundane world burns in my mind. It's glorious...". Most of the time, however, they are not heard because he turns to ash prematurely.
• When taking the book, you are asked whether you want to read it immediately. If declined, it ends up in the inventory. Acquiring the immense knowledge can therefore be done at your own pace.
• When reading the Oghma Infinium, one of three chapters must be selected to increase the corresponding skills by 5 points (but not above 100). Afterward, the book disappears:
  • Path of Might
    Block, One-handed, Archery, Smithing, Heavy Armor, Two-handed
  • Path of Shadow
    Alchemy, Light Armor, Speech, Sneak, Lockpicking, Pickpocket
  • Path of Magic
    Conjuration, Illusion, Alteration, Enchanting, Restoration, Destruction