How did you complete the Dagon Shrine quest? Looking through the wiki it appears there's no easy way. Even if you kill Jeelius you still have to fight your way out after you take Mysterium Xerxas. I tried sneak killing Harrow with the Umbra, but even in sneak mode it takes more than one hit to kill him, then everyone is alerted. I suppose I could lower the difficultly, but I'd rather not be able to easily slaughter everybody.

I think you have to fight your way out no matter what but I may be wrong, it's been only once that I did this quest. I remember my CoC slaughtered almost everyone in his path.