if you have any trouble with enthralling the moth priest Simply disable the dawnguard DLC and load your game, Save the game and then re enable the Dawnguard DLC. You will have to start the whole quest over and Anything on Serana will be lost (make sure you unequip any items on her, I lost my mehrunes razor)
but it seems to have worked for me, I just enthralled him after trying many many Times to do so. I hope I help anyone else who has had this problem. Also I disabled all of my mods just to make sure it would work Finally can continue this questline now!