So right after Harkon finishes explaining the vampire lord powers, he tells me to go talk to Garan Marethi. as I leave the room and enter the main room where the rest of the vampires are, they all just start attacking for no apparent reason.
can anybody tell me why?...has this happened to anyone else?
One of Dawnguards many EXCITING FEATURES! (bugs). You're in your vampire lord form right? If so, then this is a known bug. Don't worry, at least you'll be able to
Spoiler block out the sun! Oh wait.....we just get a bow that only blocks out the sun when we shoot a special arrow at it....when it's not too cloudy to see the sun....and when the sun is actually on the horizon (it's not when it's early morning) and it only lasts for twelve hours...oh well.
Really though, if you're in vampire lord form that's why. Just reload, switch back to mortal form and you should be fine. If not then send a followup post and we'll try to figure this out. Best of luck!