I thought this story had a good premise. I like the thought/possibilities behind what you're trying to explain: that Pensieves are part of dark magic. It's an intriguing concept to connect that to a Horcrux. However, there are a few things I'd like to point out in your first chapter.
First, you tend to drop your articles in your story. For example, you wrote: "—the other, Ben Lee, worked Florean Fortescue, " that should read he worked "with" or "for" Florean Fortescue.
Other than that grammatical issue, I would also ask what you're intending to do with this story. You kind of jumped right in without convincing your readers who your characters are. We don't know much about them except the obvious basics you gave us in your introduction. Compared to the mystery you provided in your summary, you're writing your characters pretty basic and one-dimensional. We don't really get to see them in another light or feeling.
Also, I would ask if a Horcrux can truly be made instantaneously like that. According to the HP Lexicon and the books, Horcruxes, or the splitting of one's soul, can only be accomplished by murdering another person. So I'm confused how Ben could utter a spell and instantaneously have split is soul without trying. I don't think dark magic works that way. To quote a death eater, "You have to mean it." So I would question if the events that you showed happen in your first chapter are even possible given the nature of dark magic.
However, I do think you've created an interesting storyline that has lots of potential. Good luck in your upcoming writing charades.
Author's Response: Thanks for the review. Sorry about the grammar issues-I'll fix those. The "jumping in" was because, rather than being the "actual story," it is more like a very very very very very very long prologue, so the pace slows down later on, probably in Chapter 3. I also didn't want to give away the characters immeadiately (is that how you spell it?), because I wanted the reader to gradually "discover" the character by having their actions be described, like someone would by reading journal entries.rnrnAs for the Horcrux, heheh, yeah....that was a big mistake. It was partly because I was writing it late at night and wanted to finish it and get to bed...I probably shouldn't do that. However, I think I'll try to come up with a way to solve that later on, with a time-turner.rnrnThanks for your suggestions.