I've poked around everywhere on the internet and I cannot find anyone experiencing this problem. Let me explain the whole thing and maybe someone can figure this out.

I got the Namira quest to cast 'Namira's Shroud' on all the priests in the ruins.
I went to the ruins and cast the spell on the first two priests with no problems.
Then, having taken the Atronach as my star sign, I ran out of mana.
I left the ruins to go to a temple to restore my mana.
I came back, found the next priest and cast Shroud on him.
The game froze.

Every time I go back in there and cast the spell on a priest the screen freezes up. There is no music, sound or anything and I cannot bring up the Xbox menu either. This has me extremely frustrated, as I want to complete all of the daedric quests.

If anyone can help or give any insight I would be SO GRATEFUL, OMG.
And yes, I've tried clearing the cache, several times.

Revert back to a save before you went to the ruins initially, make sure when you go there, you either have enough mana to cast the spell often enough, or, have a bunch of Restore Magicka potions on ya.