Ambush

What's going on? Don't you know an ambush when you see one?

Things are getting progressively worse for the Mages Guild. The necromancers have apparently succeeded in completing the construction of a large Black Soul Gem. It's use is unclear, but Traven fears the worst should it find its way into the hands of Mannimarco. You're to rendezvous with a group of Battlemages outside the ruins of Silorn and obtain the soul gem, at any cost.

Travel to Silorn, southeast of Skingrad, to rendezvous with your Battlemages. When you speak to Thalfin, you'll get a rundown on the basics of the situation. The entrance to Silorn is magically sealed, so you can't storm the ruins like you originally planned on doing. Instead, you'll have to set up an ambush with Thalfin, Merete, and Iver to ambush the necromancers as they exit the ruin with the Black Soul Gem. Killing their leader, Falcar, will allow you to nab the Soul Gem and return to Traven.

Of course, things aren't nearly as simple as they seem - you're going to have to set your trap by placing your teammates in positions near where the necromancers exit. It doesn't really matter where you put them, for reasons that become obvious, but if you want to play along, put Iver and Thalfin up close to the exit, while Merete hangs back a bit. When they've all been assigned a position, wait a few minutes, and a gang of four necros, including Falcar, will exit the ruins and walk your way. When they've come far enough, your Battlemages will automatically attack them.

Now, at this point, if you want to obtain the Black Soul Gem with a minimum of fuss, you'll probably need to have some way of paralyzing Falcar, who's dressed in the nicest outfit; otherwise, he'll make a beeline towards the entrance to the ruins when the battle is joined, and will likely escape there unless you have some way to stop them. The other necromancers are just garden-variety foes, and easily dispatched by your troops, but Falcar is the one that you need to kill. If he does make it back to the ruins, you'll have to follow him inside and track him down, while ignoring the slaughter behind you, where your teammates have likely turned on one another.

Silorn

Don't be too confused by the red arrows on your map. The first area here is just called Silorn, but your compass marker will be pointing you towards a door leading to Silorn Buroseli. The catch? In order to unlock the gates leading there, you're going to have to pass through Silorn Sedorseli, which leads you back to Silorn and the high ledge where the button for the gates is located. (You can jump around the gates if you have an obscenely high Acrobatics score, or the Boots of Springheel Jak, however.)

In the Buroseli, find Falcar, kill him, and grab the Colossol Black Soul Gem off of his body. Return it to Traven in the Arcane University to complete the quest.

Confront The King

The time to take on Mannimarco has arrived, and Traven knows precisely how to protect you against his enthralling mental mastery: the Colossal Black Soul Gem must be filled with a soul, and you must carry it into battle with you. Whose soul to put into the gem, though? Traven, again, knows the answer, and immolates himself to fill the gem himself. Ah, self-sacrifice. Very noble, but it'll also let you become the Arch-Mage of the guild when you finish off Mannimarco! Sweet.

Mannimarco's headquarters is in the Echo Cave, on the northern edge of Cyrodiil. You'll need to kill Bolor Savel outside in order to get the key that opens the door, but hey - he's a necromancer, so don't feel too bad about chopping him down. More of his ilk reside inside the cave, of course, along with their undead friends, so be ready for a lengthy journey into the abyss. Head through the Echo Passages to the Echo Necromancer's Chamber, where Mannimarco awaits.

Oh Mannimarco...you had me from hello.

Now, when you get close enough to Mannimarco, he'll attempt to paralyze you and chat you up, but you're under no obligation to help him do so; if you can sneak up on him and hit him with a poisoned arrow shot, you'll be able to sap him of life and force him into a battle while on the defensive. He does possess the usual necromantic spell suite, including a summoning ability, so if you can tap him with a silence effect, so much the better. Still, he's not overwhelmingly powerful, so if you can close in on him and keep him poisoned or silenced, you should be able to knock him around enough to kill him, at which point - hey, you just saved the Mages Guild!

You can grab the Staff of Worms off of Mannimarco's body, should you like; it seems to have a reanimation effect that you can use on creatures to bring them back to life and fight for you. There are also a few boss-level treasure chests in this room, so search around (including underwater) to grab everything you can before returning back to the Arcane University to report your success.

Denouement

With the Mages Guild under your control, you have a few new things to take advantage of. One, you'll be able to talk to the Mage Apprentices scattered around the University grounds and tell them to accompany you on your trips into the wilderness. They aren't particularly great soldiers, but they'll do your bidding, and will never run out, no matter how many of them wind up getting killed.

Secondly, if you speak to Julienne Fanis in the Lustratorium, she'll tell you about a magical chest in the Arch-Mage's chambers. If you place a single alchemical ingredient into it, then come back 24 hours later, you'll gain ten more of the ingredient. Best used for rare stuff, obviously, like Vampire Dust. You can only perform this once a week, however, limiting its usefulness somewhat.