I have encountered a serious glitch in the main questline of Dawnguard. After researching on the internet, I have also found I am not the only one experiencing this glitch. In the quest, Prophet, once I rescue the Moth Priest Dexion Evicus from Forebears' Holdout, he immediately begins attacking me after I let him out of his magical barrier cell. I don't have a previous save to reload, and even if I did, it would be likely he would do the same thing again. Numerous people on other forums such as Gamefaqs and Gamespot are having the exact same issue. The first attack after I let him out of that magic barrier, I had to beat him down to crouching point before I could talk to him, and once he got to the castle to read the Elder Scroll, he began attacking EVERYONE. I had to reload an autosave immediately and use my Imperial power of calming (that once a day use power) just to get him to properly read the Elder Scroll and complete the quest. However, after a minute was up, it was back to the same thing of him attacking everyone and getting beat up until he crouches. Now it keeps saying "This person is busy" when I try to talk to him. Does Bethesda even know about this glitch? It really needs to be fixed! HELP!!

When you let him out, I thought he was supposed to attack. Then you turn him into a thrall and he is no longer an enemy.

But, that other part with him attacking everyone is strange. I would say to get your illusion up to at least use Pacify and then dual cast it on Dexion if you goes hostile.

Other than that, I would say wait for a patch.

I'm on the Dawnguard's side, so how am I supposed to turn him into a Thrall?

He's meant to attack you at first, but when you've clobbered him a few times he takes a knee and apologises for attacking as he was under the Vamp's spell. It's having Serana with you that is the cause. Tell her to wait at the entrance of the cave and release the priest on your own. Works for me and quite a few others from what I've read !

When you beat him into submission, did he then give a little speech about not being in control of himself and seeing through someone else's eyes? If not, that may be the problem: the quest stages may not be updating properly.

I have encountered the same glitch in my werewolf playthrough. I havent touched that character since, will wait until bethesda fixes it I guess.

This sounds like a good diagnosis.

There's also the possibility that when you get back to the fort, Vampires are attacking, which might set NPCs into fight mode. Probably not, but it's happened to me (outside the fort).

EDIT: Re-reading your post, my suggestion doesn't sound right, actually.

Still, it's a shame that the OP is jumping on Bethesda for a glitch when the first half is pretty much how the quest is meant to play out.