This is different... I was expecting that Mistress Shadow would turn out to be Molly Weasley or even a female Hogwarts Professor... not an assassin for hire.
I am a bit confused as to what happened... but then I was just concentrating on the Harry part.
So all the guys at the bachellor party were killed? And Remus's severed head was giving to Narcisa?
Don't see how this could be a oneshot.
Had you concentrated just on Harry+Fluer, I could see this as a oneshot... but with the rest you thrown in... I have to ask, what's going, and what happens next?