Ok does anybody know when the guy come to the altar

thx

He doesn't.
Just kill him.

Actually, if you sit out of site (like, on top of the cave entrance) while stealthed, they will come out shortly after you get there first... Or, I am sure you could just go inside and WIPE THEM ALL OUT!! KILL THEM ALL!! But either way, you can either kill the guy before he gets to the alter (remember, Trevon told you to SPY!!). If you kill him first, grab the note off his corpse. If you WAIT, you will SEE him do the Black Soul gem thing and the journal update will pop up and you wont have to kill anyone (unless you want to, or derive some sick enjoyment from it...) but yeah... thats it, just sneak out of sight, and if it doesnt work, go kill everyone....

Cadaver

Didn't know that
I'll try to be more subtle next time

Could I ask when this quest appears? I thought the black soul stuff was part of the Mages Guild quests, but I've completed those. I'm Arch mage and have a 'huge black soul gem' in my inventory that I can't drop or get rid off.

I'd like to get my hands on some normal black soul gems.

Is this quest perhaps part of another guild?

Never mind I read a walkthrough and saw that I hadn't done this quest quite according to optimal. I just entered the cave when I got there and killed them all, so I missed the ritual.

Anyway I know how to create black soul gems now. I thought they had to be found.

For anyone else wondering about Black Soul gems, once a weak at the alters, you will see a "light from the heavans" on a necromancy alter. Put grand soul gems on it, cast dispel on touch or target on the alter, and PRESTO, black soul gems that will trap human souls. And really, if you are level 15, arent bandits easier to kill than Gloom Wraiths? lol, and still get the 1600 enchant points, or highest level constant effects.

As for a list of alter locations, one of the early mages guild quests, you get a note left by Falcar that someone picked up, it has the locations of a few other lairs, I think there are alters there...

Good luck, Dark Brotherhood Vampires might as well be Necromancers too, I say, lol....

Cadaver