OK, I've finished the Mage's Guild quest, and decided that it would be nice to go to one of those altars and make some black soul gems to trap high level monsters. The problem is, when I went back to the cave where you're supposed to get evidence that the necromancers are making the gems, it's gone. Does anyone know of any places where the necro altars are after the Mages guild quests are done, or am I doomed completely from a life of evil.

You should get the locations on the scrap of paper that leads you to the Dark Fissure. I can remember two Fort Linchal and Fort Istirius. I think there is also an Ayleid ruin but I forget the name.

You can sometimes retrieve Black soul gems from the corpses of Necromancer Adepts.

a high level monster cannot be trapped in grand soulgems? can anyone explain the use of black soul gems to me?

Umm, you can find this information elsewhere on these forums, and this technically counts as thread hijacking... I dunno, I guess it sort of fits with the topic.

From my understanding Black Soulgems can be used to trap the souls of humanoid characters, namely NPCs, and Dremora, which isn't possible with your average Grand Soulgem.

I found the Altars situated at the Forts before I ever joined the Mages' Guild, so I say it's a good bet they'll still be there.

I don't know if it's the same with the Dark Fissure's altar, however.