I bought Dawnguard when it was first released on Steam and finished it with no major problems at all. Since then I have installed Steam on my new PC (well it's actually the old one with a new mobo and processor as well as reinstalled Windows 7) along with my games (including Skyrim). I started a new character and have finished pretty much all the quest lines in the game except for Dawnguard (saved it for last) at lv.62 I finally started the Dawnguard storyline and sided with the Dawnguard. Everything was going fine until the point where Isran tells me Serana has come to fort dawnguard and is asking for me. He escorts me to the torture chamber where Serana should be waiting but... she's nowhere to be found... I tried several methods to remedy this like reloading an older save file but nothing seemed to help. So in the end I had to use a console command to spawn Serana there (I did not dublicate her) and I was able to continue on with the quest.
Again things seemed to go well until I got to the point in the "Prophet" quest (This is where the real problem starts) where I'm supposed to find some carriage wreckage south of Dragon Bridge... well there's no carriage there nor any dead vampire to be found... again I'm forced to use a console command to skip over that part. Next I'm supposed to head to Forebears' Holdout which is a cave southeast of Dragon Bridge. Well the odd thing is that the entrance to that particular cave does not exist in my game. I had to confirm the exact spot of the entrance from several youtube clips as well as the Skyrim Wiki and I'm 100% sure that the entrance to the Forebears' Holdout is missing from my game since I can't see it on my compass nor map and there's just a huge rock where the entrance should be located.
At this point I thought maybe some mod is conflicting with this even though I don't have any mods related to Dawnguard installed nor any that would change the landscape. I disabled almost all of my mods but that didn't help either, the entrance is still missing. Okay maybe the mods screwed up my save file or something I thoughyt. So I uninstalled Skyrim completely and reinstalled it. This time I didn't install any mods, it was pure vanilla Skyrim. I started a new character just to be sure and finished the helgen part. Then I used the "TMM 1" console command and checked my map... ALL the map markers were there except for Forebears' Holdout. I was shocked, how can this be?! Now I was running out of ideas so I tried the "Verify integrity of game cache" option from steam thinking maybe the DLC didn't download correctly. The result was that "1 file failed to validate and will be reacquired" then steam downloaded a 752 byte update to Skyrim. I thought maybe that's the file that was screwing up my game. I loaded a save file and still Forebears' Holdout was missing even on a brand new character.
I tried the "Verify integrity of game cache" again and again and each time Steam downloaded that same 752 byte update to Skyrim which did not change anything in my game.
At this point I was getting desperate so I loaded my character which was stuck during the "Prophet" quest and used the "coc" console command to teleport myself inside Forebears' Holdout. It worked! But there's one problem... the cave is supposed to be guarded by vampires but in my game the cave is empty... only the moth priest was standing there inside the barrier and no vampires to speak of... So then I just decided to give up. I tried pretty much everything and nothing helps. I have no idea what is causing this but I hope there is some sort of fix...
I was really excited for Dragonborn DLC but not even being able to finish the old DLC because of some odd glitch like this has really put me off buying it :/