I think Corprus was just born from Dagoth Ur's Blights, which were in turn born from the power of the Heart.

iirc, the rank and file ash creatures and ascended sleepers ate it. Well, really the whole cult ate from the Divine Disease in order to acquire and master it. But the Vampires didn't need to because they had fully mastered the disease and received Dagoth Ur's ultimate blessing.

Where did Corprus come from? The Heart of Lorkhan. The popular idea is that it infuses your body with more life and vigor, which unless properly mastered manifests as very specific cancer. If you look at Akulakhan close enough, you'll see that it's skin is Corprus-Flesh. Kinda makes you wonder what it would be capable of if it was operational.

Suddenly, Cancer for everyone!

This isn't really new. If you remember they said it even in the game, that the rumor about corpus is some strange guys are sitting in remote caves, cutting of their own flesh and eating it, because new flesh would regrow on it's place. New and stronger.
I think this was a random rumor opton in the game.

But technically I think the answer to my question is: Nobody knows, and there is no proof next to or against the idea of biological relatives, with the exception of that one book and the fact that having 8 kids would be a bit too much even for a dog, not to mention an elf.

Yes, they are his brothers - as you note this is reinforced throughout the game.

Every rule has exceptions, just because the book says it's usual for them to only have a few doesn't mean its impossible for them to have more.

Not to mention the Ash Vampires were born nearly 4000 years ago, whereas that book was written shortly before the time of Morrowind. It's perfectly likely some things changed in that period.

Thank you Luagar2!

Don't they all have Dagoth surnames? I could look this up, but Benadryl isn't being my friend right now. I'm pretty sure they all have the surname of Dagoth.

@Luagar, anything to suggest that they are fraternal brothers instead of just being adopted clan members? In the same way that the PC can become a family member of a Great House. 4000 years is quite awhile though, and the early members of the Great Houses were surely related, not yet adopting new members into the families.

Actually, their family names are not known. Dunmeri/Chimeri naming tradition goes: House name, personal name, surname. Like Hlaalu Athyn Llethan, Indoril Nerevar Mora, and so on. The Ash Vampires are just called Dagoth Endus, Dagoth Tureynul etc etc.

Indoril Nerevar Mora? Is that canon? I have never EVER heard that Nerevar's surename was Mora. He hass always been called Nerevar Indoril everywhere. Where is this from?

Four thousand years for elves that can live 500+ years is not that many years.... Not enough to change reproduction so greatly. But can't you see brothers more as a "band of brothers" forged in war or something like that? I seem to recall Dagoth also calling the Nerevarine his brother in one of the speeches you get as you walk up to the heart chamber... Though that last part maybe wrong.. Still it doesn't seem to me like they have a blood relationship.

One of the devs pointed it out a while back (in an interview I believe) - it's over at TIL.
500 years isn't the norm.

There's nothing to stop you from interpreting it in a 'band of brothers' manner, there is simply nothing to suggest that, which means it is unsupported and you have no basis for making the claim - Occam's Razor and all that ass well. It calls them brothers and there's no real reason to assume that isn't supposed to be taken at face value.

Dark Elves don't live 500+ years. Maybe the Chimer did, but Dark elves officially live for 130-150 years, as it was stated in an interview at TIL.
I think Dagoth calls Nerevar a brother-in-arms, and not brother, but I may be wrong. And even if he did, Dagoth Ur is a bit obsessed with Nerevar and the Nerevarine, so I don't think the way he treats them is how he treats everyone.

Yes, it was a rumor. I remember it now.

I once took a look at the TES timeline, did the math and came to the conclusion that Nerevar was around 600 by the time of his death. As for House Mora, it was mentioned in the sermons. I never knew that Nerevar was of Mora. I guess it was a minor house or a clan and probably never rose to power.

Still seems more logical to me then Dagoth having god knows how many brothers... Seeing as everything we know seems to suggest elves don't reproduce or have never reproduced in great numbers. And even if Elves have human life spans then still going from many children to just two or three is illogical unless there were massive social changes that created such needs which there never have seem to be, especially not along all elves (seeing as Altmer have the same reproductive speed as Dunmer)

I am tempted to agree with you, but I have doubts because:
a, The ash Vampires could very well be Dagots brothers even if the Chimer didn't reproduce in such a huge number. How? Well, nobody said anything about having the same mother! Their father could have very well have many wives. We know that the orc chieftans do, and they were aldmer before their change - so this can be something they kept from the aldmeri culture. Or, based on what we know about Almalexia and her numerous husbands she kept to satisfy her "ever changing female needs", I think we can say that the Chimer knew what divorce is, and practiced it too, so Lord Dagoth yould have had many wives even if he didn't have them all at once.

b, We do know that, as War_Psyence said, that Nerevar had to be around 600 at the time of the battle of Red Mountain. We also know that the Dark Elves officially live only a little more than 100 years - altaugh some books seem to contradict this. Another thing we know, is in a book I found in Skyrim, in the temple of Mara, in Riften. (I am sorry but I don't remember it's title). It was about the Aedra and the Daedra, and how the different races believe that the world, Nirn was made. In that book they claimed that from the begining of the times the residents of Nirn get mored moved away from their Aedric spirit, and their blood is "thinning". This means that the elves now have shorter lives than they had before. Also, we Know that Hgh elves being obsessed with the protection of pure aldmeri blood live longer than the rest of the elves.
I would say based on all this, the Chimer, technically being Aldmers with a different religion, must have had longer lives - maybe 700-800 year long, but this thinning of blood shortened the Dark Elves lifespan.
Through 700 years of advlthood one can have 8 children. Nothing says the Ash Vampires were of similar age! Dagoth Uthol even says that he can hardly remember Nerevar at all, which might suggest that he was much younger than the rest. Or he REALLY didn't pay any attention to politics.

c, Elves reproduce based on the social and environmental stress they face. I think I read this too, in one of the interviews at TIL.
This basically means that when there are many elves, the birthrate is low. When their number is low, the birthrate jumps up. Nowhere, with the exception of Vivec's Sermons and the Five Songs of King Wulfhart do we see any suggestion of when did the fight between the nords and the chimer begin. Vivec says that it began after Nerevar began to rise, not too long before the Battle of Red Mountain. The Five Songs however suggest that they have been fighting since about the chimer arrived to Morrowind, only the fights stopped after the nords lost their king and became un-united. Basically that book says that there were 2 nord-chimer wars.
I would also like to point out that Vivec isn't exactly the most reliable of narrators in his Sermons. I am not saying that everyting in the Sermons are lies, but after Vivec claiming that he was born as a god from an egg, Dagoth Ur has been sitting under Red Mountain since the begining of times, and Vivec had a bunch of children from Molag Bal while his head was traveling with Nerevar, in his backpack...well... I have a difficult time to take him serious.
My point is: it seems fairly possible that the chimer had been in a constant state of war from the moment they arrived to their new home, and for years before Dagoth ur and the rest of his boys were born. Seeing where Kogoruhn is, I would say they were pretty much in the middle of the fights with the nords and the dwemer. This could have led to a dangerously low population in Great House Dagoth, and therefore to an extremely high birthrate at that time, which could explain this many children. But this is only my theory, of course.

Edit: my spelling really seems to svck today. Sorry about that! =_=;

All of that makes sense... So I have no idea what to think about this anymore

There are a ton of references to House Mora. Vivec "appeared as a visitation in the ancestor alcove of House Mora, whose rose-worn prince of garlands was a hero against the northern demons" in the 22nd Sermon, Douglas Goodall said "For instance, Indoril Nerevar's full and formal name would be Serjo Indoril Nerevar Mora", The Real Barenziah has Symmachus saying that Nightingale is probably from House Mora and that the clan has human blood in it, and finally Interview with Three Booksellers says "Dorisa: Barenziah was born into the R'Aathim, that is true. So were the King Hlaalu Athyn Llethan and the Empress Katiarah. The R'Aathim are a clan, or perhaps "family" is a better word. They were once part of House Mora, but they were adopted into House Hlaalu. House Mora is no more."
It's all very interesting.

There have been quite a few times throughout my fan-fiction where I had the Dagoth brothers mention how unusual, and at times annoying, it was to have so many siblings. At one point Araynys and Voryn mentioned that their parents had gone to an enchanter for a sterility amulet, but the enchanter made a mistake and had instead ended up giving them a powerful fertility amulet. All eight of the brothers describe "Mother Dagoth" as a saint for putting up with all of them.

This is of course my own creative twist on the lore that Bethesda has given us.

This made me think of something. Aren't we all forgetting the possibility that the Dagoths had a father who either slept around a lot or took multiple wives? If elven women don't have a lot of kids, who says it wasn't a case of multiple elven women impregnated by the same guy?

Yeah. I find the possibility of Papa Dagoth simply having most of his children from a harem likely.

Fyr calls the clones he grew through his experiments with corprus his 'sister-wives'.

Is it possible Dagoth's brothers are similarly products of corprus?

They were around before corpus, so no. I also find it unlikely that Voryn or his dad grew them in a test lab somewhere. You'd think that there would be some books written about it, especially with people of such importance in that time. There would have had to have been a reference somewhere.

Their are a number of Dunmer in Skyrim that were alive for the red year and will tell you about their experiences of that time, another who was born around that time, as well as a few references to Dunmer aging in game, all of which seems to invalidate that interview.