I thought this was a kickass chapter! Best in a while, definetly, and I can't wait for the next one!
I disagree with most of you on 2 points though...
1st: I don't think the dark creature has anything to do with the curse mark. I think it's the first stages of "synching with gedo mazo", or whatever madara called it. From what we saw in nagato, it clearly either requires, or generates a significant degree of darkess of soul, so this situations was probably engineered by madara, as the first step in making sasuke his tool by plunging him deeper into the dark, while distracting naruto so that he doesn't interfere.
2nd: Having said that, I don't think the evil chacra doesn't necessarily mean some new awesome power. The gedo mazo will eventually grant power, to be sure, but as of yet, all that could be happening is it's looming presence lurking closer to sasuke's soul, drawn by the murder living there.
Thoughts?