After I finished playing through the Soul Cairn part of Dawnguard, I felt like we learned more about necromancy in that quest than we did throughout the entire Mages Guild questline in Oblivion, so I went back and looked at the Oblivion quests to see how it all fit together.

In Oblivion, the Mages Guild, or at least Raminus Polus, doesn't seem to have any idea what black soul gems do, or how they are made. One of the quests is doing research on the subject and he says:


It seems rather unlikely to me that a Guild that has had necromancers in it for years, in an Empire where necromancy is legal, wouldn't know what black soul gems do. Unless the necromancers just kept their research secret?

Then Dawnguard comes along, and we see that Serana and Valerica are necromancers. Valerica (probably?) predates Mannimarco, but is definitely a skilled necromancer, and has been doing research for years- into the Soul Cairn. She describes how undead summoned from the Soul Cairn are much more powerful than ones raised by a necromancer, and, if I recall correctly, refers to this sort of necromancy as a lost art. And we also see that black soul gems clearly can be created independently of Mannimarco, because they can be found and created in the Soul Cairn. Are there any other ways to make them?

Can all of this be fit together somehow?

Maybe once Mannimarco ascended to godhood, necromancers began worshiping him and using him as a source of power rather than having to make much riskier deals with the Ideal Masters?

I have not played as much of Morrowind as I should, so I'm not sure how that might fit in here either, any thoughts?

Or maybe Mannimarco had already become an avatar of Ideal Masters invading Mundus and disturbing Arkay.

He is called "the high priest of maggots" by http://www.imperial-library.info/content/nu-hatta-sphinxmoth-inquiry-tree after all.

Maybe the black soul gems only surfaced recently due to a pact of some kind in/near Oblivion events between Necros and ideal masters (both would be very enticed by such a pact in their line of work). Perhaps black soul gems are even tied to the Soul Cairne somehow? As for Serana and Valerica, vampires are known to use necromancy to some degree, its not news.

As for other ways to craft black soul gems, ill refer to the part in Oblivion where you can craft some at several altars under a night known as Shade of the Revenant, which im willing to bet has something to do with the Soul Cairn, since the light produced on this night seemed similar to that permanent glow in the soul cairne.