A Plot Revealed
Traven seems to have calmed down when you next speak to him, but he tells you that the Council of Mages is still in an uproar over the news of Mannimarco. Mannimarco obviously has it in for the Guild, but there haven't been any demands from the Necromancers. So everything is still in flux.
In the meantime, Traven hasn't heard anything in a while from Jeanne Frasoric in Bruma. This has him worried in light of the Count's news of Mannimarco. So he would like you to head up to Bruma to make sure everything is OK.
Nothing seems to be amiss when you arrive in Bruma, but that impression changes as soon as you step into the Guildhall. For one thing, there isn't anyone around except some undeads (who are not happy to see you). Secondly, the place is a burning wreck. I hope you don't need anything from the Bruma guildhall.
After disposing of the undeads that you can get to, you'll need to go down to the basement. You'll encounter even more undeads in the basement. When you reach the other side, head upstairs. At some point along the way, you should hear someone calling for someone else to come out of hiding. That would be Camilla Lollia, a Necromancer. She's looking for J'Skar, who is invisible (again). You'll need to dispose of Camilla, but it's possible to take her out before she starts in on her evil villain rant (I realise that it helps tell a story, but why do they have to do it so often?).
In any event, once you've taken care of business, J'Skar will turn visible and tell you what happened. Mannimarco attacked the guildhall, killed everyone, sucked out Volanaro's soul, and said that he was going to destroy the Mages Guild. J'Skar says that he mentioned something about Echo Cave and then runs for the Imperial City. You'll find him in the lobby of the Arcane University tower when you get back.
After disposing of whever undeads may still remain, feel free to loot whatever is left. You should still find a few potion ingredients, alchemical stuff and maybe a few golds. The Bruma Guildhall is gone and, absent a plug-in or mod from Bethesda or some kind fan, will not be restored.
Report back to Traven on what you found. He'll take a few days to get his act together, but you should talk to Polus for a promotion to Wizard. Polus will also teach you the spell, "Wizard's Fury". This is a combo Fire-Frost-Shock damage spell that's tied to your level. The higher your level, the more powerful the spell's effects. It dishes out a total of 19 points of damage at 1st level, up to a maximum of 210 points at 30th level or higher. If you haven't been working on your Destuction skill, you may not be able to cast it.