Necromancer's Moon

Time to learn more about those mysterious Black Soul Gems that you discovered in your recommendation quests. Raminus wants you to speak to Tar-Meena, whom you may have encountered in the game's main quest if you've been playing through that. She may know something about the function of the Black Soul Gems; apparently no one's ever bothered to ask her.

After finding the Mystic Archives, talk to Tar-Meena, who'll tell you to track down a book called Necromancer's Moon. She doesn't know precisely where it is, but your compass point will, as usual, save the day; look for the book on a small table near one of the desks here. Read it, then add it to your inventory. After speaking on its mysteries with Tar-Meena, return to Raminus, who'll point you towards Bothiel, the caretaker of the Imperial Orrery, which is as of yet still locked to you. You should be able to find her walking around the Arcane University, however.

Speak to Bothiel to obtain the Hastily Scrawled Note, which points out a list of four necromantic sites, complete with altars capable of transforming Grand Soul Gems into Black Soul Gems. Hmm. Raminus desires you to explore one of these ruins; the ominously named "Dark Fissure". It's right near Vahtacen, so at least you won't have a lengthy journey to get there.

The Dark Fissure

Note the appearance of the altar outside the cave here. If you head inside and kill the first necromancer inside the door, you should be treated to an explanation of what it's for if you read the Handwritten Note on his body. Apparently, if you place a Grand Soul Gem in an altar, wait until the spectral light shines on it from the skies (which happens one day a week), then cast a Soul Trap spell on the altar, you'll obtain a Black Soul Gem. Curious.

The only good necromancer is a dead necromancer.

With the Note in tow, you can leave the cave and return to the Arcane University if you wish, but you can also, of course, search through the Dark Fissure. There are a set of Tunnels beneath the first level. You can find the key for the door in a chest in the necromancer's sleeping area. Beneath the Tunnels is the Inner Sanctum, where you'll find more necromancers and at least one boss-level chest. Well, it's really a coffin, but it's still got good loot.

When you return to Raminus, he'll offer up another advancement, and tell you to seek out Arch-Mage Traven himself, who has a special assignment for you.

Liberation or Apprehension?

Mucianus has seen better days. There's nothing you can do for him at this point.

Traven has a delicate matter at hand: one of his spies in the necromancer cult, one Mucianus Alias, is suspected of being a double agent by the Council, which has sent a group of Battlemages to his last known location with the intent to bring him back into the Guild's safekeeping. Traven asks you to do pretty much the same thing, but to cut off the Battlemages before they arrive, and thus ensure that Mucianus isn't mistreated.

Proceed to Nenyond Twyll with all due speed. You'll encounter a battlemage named Fithragaer just inside the entrance whom, before he meets his hilarious fate, will tell you that the necromancers wiped out the rest of the battlemages and retreated further into the ruins. If you chase them through the first portion of the level, you'll encounter one Mariette Rielle just after you reach the Riellesel area. It would appear that Mucianus is now zombified himself, and thus won't be returning with you to the Arcane University. She does make some kind of reference to "The Master", which bodes well for some kind of Manos-esque arch necromancer at the end of this questline.

At the far end of the Riellesal zone, you'll come across a Necromancer Adept, who'll guard access to a pushblock on the wall. Hitting it will reveal Mucianus Alias, who has indeed been transformed into the walking dead. Remove the head or destroy the brain, if you wish, but if you don't want to kill him, you can just leave him be; he won't attack you. Report his fate to Traven to complete the quest.