Okay, so today I just started playing the DLC, "Dawnguard", and I got to the point where you have to rescue the Moth Priest from the Vampires. I did that but I saved after I did a bad thing without realizing it - I made him a hostile. So when I continue on with the question, "Prophet", I can't complete it because when I enter Fort Dawnguard, Isram and the Moth Priest and other Dawnguard soldiers attack the Moth Priest on site all the time. I can't complete the quest due to this stupid glitch but I think I have a solution, I need a Master Illusion spell called "Harmony" or any other spells to make hostiles friendly for a period of time. I'm level 81 by the way, all perks used and none in Illusion or any other type of magic. So that brings my to ask two questions:

1.) Where can I get buy these spells (Master rank)

2.) And will I be able to use them?

If not, than can I enchant armor so Magika is increased or Illusion spells cost less to use, than would that work? If someone can help me out, I'd appreciate it very much!

And not like this is important but my character is primarily a Thief/Assassin class so nothing towards Magic was increased.

Drink a fortify illusion potion and dual cast the most powerful 1 hand calm spell.

Find a save before you enter the cave. Save then. And enter. If he isn't hostile, good. If he is, reload to that save.

Okay, so where do I buy the spell? I'm assuming the College of Winterhold? And if so, where?

Also, how large does the number have to be on the Fortify Illusion potion? I'm a huge Alchemy buff with recipes but not numbers xD

Yes the CoW. Find a Dunmer named Drevis. If Illusion is at 100 he will give you a quest. Complete that, then he will sell it.

That's the thing, I don't have one. I don't realize I'd have a problem with this because when I first played Dawnguard, I didn't have this problem and I sure wasn't aware of this glitch. And this glitch reminds me of one when I played as a Nord when I went to Apocrypha in the Dragonborn DLC I overpowered my weapons so the damage was over 1,000,000 and I could kill Miraak with one hit but, it would glitch the game because he wouldn't move in the center of the circle after that and the damage on the weapons was literally, game-breaking. So my only option was to delete the save and start over :/

Alright, thanks!

I'll be back and search for him!

Which is why you save before going into caves and before accepting quests.

Your potion will need to fortify illusion by at least 60 percent. Harmony affects NPCs and creatures up to level 25; the Moth Priest will be Level 40 with you being at Level 81.

If you don't have much magika, you may find, as I did, that you don't have enough to cast Harmony (or master level spells). If you do find this, get yourself some gear to reduce the cost of casting illusion spells. Eg circlet, necklace, ring. Either enchant yourself or buy.

What he said.

Bringing Serana along to save him causes him to remain aggro, even after putting him down and talking to him or calming him. You can work around dealing with him for the quests by calming him and such but if it's not too late to reload and tell her to part ways before entering the cave that he's held captive in then I would recommend doing so to save headaches later.

Oh and if you have Dragonborn, the Bend Will shout doubles as a powerful calm spell on humans

I have never had this bug and I always take serana with me.

The bug's related to where the Moth Priest spawns when you enter the cave. If he spawns outside the magical barrier where the vampires are imprisoning him, the glitch can happen. I don't think having Serana with you matters one way or the other.

Everyone experiences the many bugs differently but I have tested this thoroughly in my game and noticed that Serana is the cause of this bug for me...if you part ways before entering the cave then the quest updates fine and the priest doesn't remain aggro but the 3 times I tested with Serana, the quest doesn't update properly and he remains aggro even later on in Fort Dawnguard.

Maybe she doesn't cause the problem for your game but it's worth a try in his game as it might be behaving the same as mine.

I've played through Dawnguard with two characters - on both of them Serana was with me as a follower. The first character experienced the glitch - I didn't know about it at the time. Dexion was hostile to me for the entire game, and I had to use the console to complete the quest. the second character didn't experience it; with her, I saved before entering the cave, and when I entered I used the tfc console command to unlock the camera and do a "flyover" of his area when I did enter, to make sure he was where he belonged.

They really should have patched this up before calling the game "finished". I got around it since I'm on the PC, but if you're playing on the 360 or the PS3, it would be game breaking if you didn't have a previous save.

Okay, that helps a lot more. All I need to do is actually find the spell "Harmony". I found the Expert Illusion spell before "Harmony" (I forgot what it's called lol).

Having multiple save files is always a good idea, especially with a game as huge as Skyrim. Hopefully you're able to get past this part and don't have to abandon the entire dlc due to this bug and not having a backup save file. During my last playthrough, I had to repeat this quest at least once due to this same bug, but luckily it is now fixed with the unofficial patch.

I experienced this glitch too. Really gamebreaking glitch to have in a released game. Luckily, I had a previous save I could reload that only set me back 30 mins or so.

i did it if you are a imperial just use the voice of the emperor power on him if not and you have the dragonborn dlc get far enough in the dragon born dlc that you can tame dragons and use that shout on the moth pries that is what worked for me when i was doing this quest for my friend since it glitched up on him as well

Dont use the master illusion spell. dual cast the expert spell and it is more powerful.