11: Necromancer's Moon - The Elder Scrolls IV: Oblivion
You need to speak to Tar-Meena in the Mystic Archives to learn about Black Soul Gems and their power. The Mystic Archives are located on the grounds of Arcane University. Inside, ask Tar-Meena about Black Soul Gems to learn of a book called 'Necromancer's Moon.' This book, Tar-Meena explains, contains the information you are looking for.
Look for a blue book on the table behind the desk that is to your right upon entering the Mystic Archives. This is Necromancer's Moon, so take it into your inventory and then speak to Tar-Meena again. Return to Raminus Polus in the Arch-Mage's Lobby and tell him of the book. He points you to Bothiel, who will surely know something about the 'Shade of the Revenant' mentioned in the text.
Locate this Bothiel and ask her about the Shade of the Revenant. Bothiel gives you a note that Falcar dropped when he himself visited the university to ask about the Shade of the Revenant. Speak to Raminus once more and select the 'Shade of the Revenant' dialogue option to have him mark Dark Fissure on your map. Now to travel there to see what you can discover.
Make your way to Dark Fissure, which is located far east of the Imperial City.
The Dark Fissure cave is far east of the Imperial City, southeast of Cheydinhal. The hill that the altar sits upon may seem a little steep hill, but use the patches of grass along the side to help you ascend. Once you've made it to the top, enter the cave to spot a Worm Anchorite.
Kill the Necromancer inside the Dark Fissure cave and remove the note from his corpse. The note describes the process that the necromancers use to acquire Black Soul Gems. Once you have this note, there is no reason to venture further into the cave. Return to Arcane University and speak to Raminus Polus there. Afterwards, ask him about your advancement within the guild to be promoted to Magician.