Oblivion:Necromancer's Moon

Quick Walkthrough
# Talk to Raminus Polus. He wants you to speak with Tar-Meena about Black Soul Gems.
# Speak to Tar-Meena, who can be found at the Mystic Archives. She tells you to find a book called ''Necromancer's Moon''.
# Find the book in the lower room of the library on a small table. Take it back to Tar-Meena.
# Bring the book back to Raminus. He'll tell you to talk to Bothiel.
# Talk to Bothiel. She gives you a hastily scrawled note which you should take back to Raminus.
# Raminus tells you to go to Dark Fissure Cave at the next Shade of the Revenant to observe the ritual of creating Black Soul Gems.
# There are three ways to learn about the ritual:
#* Waiting: Just stay outside the cave until the ritual occurs. It happens one night per week at midnight (a light will shine down on the altar). Just observe.
#* Fighting: Explore the cave and find a Worm Anchorite. He will be the first person you encounter in the cave and will be immediately visible to you on the left path as soon as you enter the cave. Kill him, then search his body to find a Handwritten Note explaining the ritual (you can also pickpocket the note).
#* Doing: Cast Soul Trap on the altar yourself when the time is right. This will trigger you "seeing necromancers perform the ritual".
# Once you've discovered the ritual or retrieved the note, return to Raminus.

Detailed Walkthrough
Tar-Meena
After completing Vahtacen's Secret, Raminus Polus will tell you that he needs to find information on Black Soul Gems from Tar-Meena. Just head over to the Mystic Archives. When you first talk to Tar-Meena, she'll rage a bit about how she's got too much to do already, but once you mention Black Soul Gems specifically she agrees to help you out. Tar-Meena instructs you to find a book called ''Necromancer's Moon'' and mentions that Falcar has borrowed it. The book can be found on a small table in the lower part of the Library. Take it back to Tar-Meena and she'll tell you a little more information on it, as well as on an interesting event called Shade of the Revenant, before sending you back to Raminus.

Bothiel
When you speak to Raminus, you find out that he's clueless on what Shade of the Revenant actually means, but he does take a wild guess. He thinks it may be some sort of celestial event. To that end, Raminus sends you to speak to Bothiel - the guild's resident expert on such things. Bothiel can usually be found either wandering the university grounds, or in the lobby. Sadly, she doesn't have any answers for you, but interestingly enough remembers Falcar - the mage that tried to send you to your doom during the Cheydinhal Recommendation quest - asking about the same thing. Bothiel then gives you a hastily scrawled note that he left behind. The note mentions four sites that don't mean much yet, so head back to Raminus to give him an update.

Dark Fissure
After you tell him about the note, Raminus suggests that you visit Dark Fissure Cave (one of the sites from the list) at the next Shade of the Revenant. The cave is just south of Cheydinhal. Once you reach the cave you will find an altar set up, but you need to wait for the correct night to witness the actual event. On one night per week, a purple light shines down on the altar indicating that the necromancer rituals can be performed.
You can wait to see the ritual occur. It always occurs for one complete day (from midnight to midnight). On the correct day, you can witness necromancers completing the ritual, thus advancing the quest.
The Shade of the Revenant actually occurs every eight days; specifically, it occurs on the 1st, 9th, 17th, 25th, 33rd, etc. days of the game, using the "Days Passed" statistic on your character's statistics journal page, provided that you have not spent a day in jail (see here for more details).
It occurs on the same day at all four locations.
If there is no light when you arrive, you can use the wait command to wait 24 hours at a time. Or you can calculate when the next Shade of the Revenant will occur, go complete other quests, then return on the correct date.
When waiting for the purple light, do not stand directly in front of the altar; back up so you can see the altar and a fair portion of sky. The purple light is very soft by the time it gets to the altar, and if you are standing directly in front of it you may not realize it is shining and wait right past it. Also, doing this at night will help.
If you don't feel like waiting, enter the cave and find the Worm Anchorite who will be on the left as soon as you enter the cave. The quest merely requires you to acquire the Handwritten Note he possesses so you can either kill him and loot his body, or sneak up and pick his pocket. Either way, the note is sufficient proof for Raminus; you do not actually need to witness the ritual.
The final way is to wait for the Shade of the Revenant as described above and then do the ritual yourself. For details on the procedure, see the page on Black Soul Gems. Basically, you need to place an empty Grand Soul Gem in the altar and then cast a Soul Trap spell on the altar. Even if you do it yourself, this will trigger the message about "seeing necromancers perform the ritual". This can be done at any of the 4 altars mentioned in Falcar's note.
Once you've completed any one of these options, head back to the Arcane University, and speak to Raminus for advancement. He'll also tell you that Arch-Mage Traven will be giving you your duties from now on.

Notes
The hastily scrawled note that directs you to Dark Fissure lists three other locations: Fort Istirus, Fort Linchal, and Wendelbek. There are Black Soul Gem altars at all of these locations. All of the altars can be used to make Black Soul Gems, but Dark Fissure is the only place where you can witness necromancers making them.
Ask Raminus for a promotion after this quest, as it is possible to move on to Liberation or Apprehension? without actually being promoted.
  An achievement will be unlocked upon being promoted to Magician. You can miss out on this achievement if you don't ask Raminus for a promotion (see note above).

Bugs
 
 This can be fixed with the console command  .
 
One solution is to uninstall the Orrery download; within 24 hours Bothiel should reappear in the Arch-Mage's Lobby. Complete Necromancer's Moon, then reinstall the download.
 

Journal Entries