Information At A Price
Talk to Traven again. Count Hassildor has some important information that he won't give to anyone except you. Why you? Only the Count knows. So hop on over to Skingrad and talk to Hal-Liurz, the Count's new steward since you were kind enough to open the position a while back. She'll go fetch the Count, so just hang tight while she does that.
From your last encounter, the Count believes that you can be trusted, at least more than the other members of the Guild. He has some information for you, but you need to deal with a couple of nuiscances before he'll give it to you. Nothing for nothing, after all.
Nuiscance number one is a nest of vampires that have taken up residence in Bloodcrust Cavern. He's not exactly sure why they are here, but he wants them exterminated. Nuiscance number two is a group of vampire hunters who have come to town. He can't give you much information about them as he has not expressed any interest in order to keep from having their attentions drawn to the castle, but you can ask around town. He wants them gone by any means necessary, including killing them. However, if you take that route, you're on your own with the guards since he cannot intervene on your behalf.
It's easier to deal with the second problem, so hop on down to the town proper and start asking around about the vampire hunters. They're staying at the Two Sisters Lodge. Mog gra-Mogakh, the owner, will tell you that they come in to eat in the afternoons and go to bed around midnight. Aside from that, you're on your own. You can scour the town to find the location of two of them. Eridor, the leader, likes to hang out by the Chapel and is the one who can give you the most information. The others will just refer you to Eridor.
There are basically three approaches that you can take to dealing with these two problems: take out both groups by yourself, work cooperatively, or just deal with the vampires.
Taking out both groups is the hardest approach, but is more lucrative than just dealing with the vampires. Head off to Bloodcrust caverns and take out the vampires. It's a fairly straightforward dungeon crawl and shouldn't present any problems if you go prepared (vampires are a known quantity, after all). Taking out the vampire hunters is a tad more difficult since you'll need to take out each hunter individually and in circumstances where they will not be able to raise an alarm. Even if you escape the attention of the guards, you will probably not escape the attentions of the Dark Brotherhood since this approach is cold-blooded murder. Carsten is two levels above yours; Eridor and Vontus Idolus are one level higher than you; and Shamar is the same level as you. Only Vontus is far enough away from town that you can take him out with little fear of interference from the guards. Eridor will be most happy to provide you with their locations if you ask.
Just dealing with the vampires is the simplest approach, but it takes the longest. Again, you need to clear out Bloodcrust Caverns. Then simply tell Eridor that you have dealt with the vampires and wait for them to go check it out. They'll leave town some time after.
Letting vampire hunters be vampire hunters is a nice compromise. It's easier than taking out both groups by yourself and faster than just dealing with the vampires. Tell Eridor about the vampires at Bloodcrust Caverns and then sit back and let nature take its course. They'll try to clean out the entire nest (and most likely die in the process). Then you can waltz in, clean up what's left of the vampires and carry away the loot from both sides. If, by some wild stretch of the laws of probability, one or more of the hunters should survive, they are now in a place where you can take them out without interference from the law. Or, they should leave town on their own if you leave them alone.
Now go talk to the Count again. He tells you to let the guild know that Mannimarco has returned. He won't give you any more information than that.
If you poked around the Mystic Archives, the name should be very familiar. There is a book in a display case called "Mannimarco: King of Worms" which tells you of the split between Mannimarco and Galerion. "Daggerfall" players will recognise the King of Worms from the Main Quest in that game. "Redguard" players may recall N'Gasta talking about him. And there were brief mentions of him in some of the books of "Morrowind" ("Where Were You When The Dragon Broke?" being the one that springs first to mind).
When you report back, Traven goes semi-ballistic over the news and won't have any more tasks for you for a couple of days while he digests this information. You don't even get a promotion for your troubles. So go find something to do until Traven is ready to talk to you.