Wow, I don't think I've seen this in your gallery before... I really like this one!
Okay, forgive me if I'm wrong, but looking at it, especially in the castilene, and "floating", it looks like a woeful haint and it has a feminine feel to me... like she is lost and looking for something....or someone. Very creepy if you think about her, well "it" that way.
Also I've been under a rock when it comes to Harry Potter so I don't know what a dementor is. And googling.......Yep, that's a Dementor! O.O Great job!
((I still say it can double for a haint. )) At any rate, it's beautiful work.