As a crazed expert on the Elder Scrolls, Septimus wrote the book, Ruminations on the Elder Scrolls, which attempts to describe the nature of the prophetic scrolls with obscure metaphors. He wrote the book while he was still a member of the College of Winterhold, departing after having read one of the Scrolls and becoming irrevocably mad.
Looking for Dwemer artifacts, he wandered out on the small glacial islands north-west of Winterhold. Here, he proceeded to attempt to access a Dwemer contraption. He was believed to be driven insane by his own genius, and the long periods away from people, although he admits reading the Elder Scrolls caused his madness.
Urag gro-Shub supplies the Dragonborn with reading material on the Elder Scrolls, one of which is the book authored by Septimus. Suspecting that he is the most likely to know where an Elder Scroll may be, the Dragonborn locates him north of the College of Winterhold, at an outpost.
Once found and spoken to, he sends the Dragonborn to Alftand with a Lexicon, a device created by the Dwemer that allows one to safely transcribe the text of an Elder Scroll without going blind or insane.
Exiting the outpost reveals that Daedric PrinceHermaeus Mora seeks the object within the Dwarven Mechanism and has only tolerated Septimus' involvement with it because it served his own plot. The Dragonborn can choose to aid Hermaeus or decline the Lord's offer. Neither option directly affects the outcome of unlocking the mechanism.
Once acquiring and giving the blood samples to Septimus, he opens the contraption in his outpost and is extremely disappointed to not see the Heart of Lorkhan. He then disintegrates into a pile of ash as he approaches the Oghma Infinium. The Dragonborn can then take the Oghma Infinium.
"When the top level was built, no more could be placed. It was and is the maximal apex."
I heard you know about Elder Scrolls. (Only during Elder Knowledge)"The Empire. They absconded with them. Or so they think. The ones they saw. The ones they thought they saw. I know of one. Forgotten. Sequestered. But I cannot go to it, not poor Septimus, for I... I have arisen beyond its grasp."
What brings you to this remote place?"The ice entombs the heart. The bane of Kagrenac and Dagoth Ur. To harness it is to know. The fundaments. The Dwemer lockbox hides it from me. The Elder Scroll gives insight deeper than the deep ones, though. To bring about the opening."
You have an Elder Scroll? Here?"I've seen enough to know their fabric. The warp of air, the weft of time. But no, it is not in my possession."
So, where is the Scroll?"Here. Well, here as in this plane. Mundus. Tamriel. Nearby, relatively speaking. On the cosmological scale, it's all nearby."
Are you... all right?"Oh, I am well. I will be well. Well to be within the will inside the walls."
Can you help me get the Elder Scroll or not?"One block lifts the other. Septimus will give you what you want, but you must bring him something in return."
Don't make me hurt you. Just tell me where it is."Oh, a brutish one. Septimus has no fear of you. But as one block raises another, perhaps ourselves could help us each."
What do you want?"You see this masterwork of the Dwemer. Deep inside their greatest knowings. Septimus is clever among men, but he is but an idiot child compared to the dullest of the Dwemer. Lucky then they left behind their own way of reading the Elder Scrolls. In the depths of Blackreach one yet lies. Have you heard of Blackreach? "Cast upon where Dwemer cities slept, the yearning spire hidden learnings kept.""
Where is this "Blackreach"?"Under deep. Below the dark. The hidden keep. Tower Mzark. Alftand. The point of puncture, of first entry, of the tapping. Delve to its limits, and Blackreach lies just beyond. But not all can enter there. Only Septimus knows the hidden key to loose the lock to jump beneath the deathly rock."
How do I get in?"Two things I have for you. Two shapes. One edged, one round. The round one, for tuning. Dwemer music is soft and subtle, and needed to open their cleverest gates. The edged lexicon, for inscribing. To us, a hunk of metal. To the Dwemer, a full library of knowings. But... empty. Find Mzark and its sky-dome. The machinations there will read the Scroll and lay the lore upon the cube. Trust Septimus. He knows you can know."
What do I do with this sphere?"The deepest doors of Dwemer listen for singing. It plays the attitude of notes proper for opening. Can you not hear it? Too low for hearings?"
What do I do with this cube?"To glimpse the world inside an Elder Scroll can damage the eyes. Or the mind, as it has to Septimus. The Dwemer found a loophole, as they always do. To focus the knowledge away and inside without harm. Place the lexicon into their contraption and focus the knowings into it. When it brims with glow, bring it back and Septimus can read once more."
What is an Elder Scroll?"You look to your left, you see one way. You look to your right, you see another. But neither is any harder than the opposite. But the Elder Scrolls... they look left and right in the stream of time. The future and past are as one. Sometimes they even look up. What do they see then? What if they dive in? Then the madness begins."
What do you want with the Elder Scroll?"Ooooh, an observant one. How clever to ask of Septimus. This Dwemer lockbox. Look upon it and wonder. Inside is the heart. The heart of a god! The heart of you. And me. But it was hidden away. Not by the Dwarves, you see. They were already gone. Someone else. Unseen. Unknown. Found the heart, and with a flair of the ironical, used Dwarven trickery to lock it away. The scroll will give the deep vision needed to open it. For not even the strongest mechinations of the Dwemer can hold off the all-sight given by an Elder Scroll."
After transcribing the lexicon:
I've inscribed the lexicon."Give it, quickly. Extraordinary. I see it now. The sealing structure interlocks in the tiniest fractals. Dwemer blood can loose the hooks, but none alive remain to bear it. A panoply of their brethren could gather to form a facsimile. A trick. Something they didn't anticipate, no, not even them. The blood of Altmer, Bosmer, Dunmer, Falmer, and Orsimer. The elves still living provide the key. Bear you hence this extractor. It will drink the fresh blood of elves. Come when its set is complete." Why are you so eager to open the box?"The box contains the heart. The essence of a god. I have devoted my life to the Elder Scrolls, but their knowledge is a passing awareness when compared to the encompassing mind of divinity. The Dwemer were the last to touch it. It was thought to have been destroyed by the Nerevarine, but my lord told me otherwise."
Who is your lord?"The Daedric prince of the unknown. Hermaeus Mora. I thought there were no secrets left to know. Until I first spoke to him. He asks a price -- to work his will. A few murders, some dissent spread, a plague or two. For the secrets I can endure. In time, he brought me here. To the box. But he won't reveal how to open it. Maddening."
After gathering the blood:
I've brought all the blood you require."I can almost... hear them. I feel their life energy. Come, I will make the mixture."
After unlocking the box:
"What is this... it's... it's just a book?! I can see. The world beyond burns in my mind. It's marvelous...."
The name Septimus Signus is derived from Latin. In Latin, the neuter form "septimum signum" means "the seventh sign." His name may be inspired by the seventh seal, the words of sign and seal being similar in the language. In the Abrahamic apocalypse story, the opening of the seals (detailed in the Book of Revelations) leads up to more cataclysmic events and judgements.
Skooma can be found in his cupboard where he keeps his books.
His ramblings seem to indicate that he believes the Heart of Lorkhan is within the Dwemer lockbox.