This seems like a rip off of the "Stranger in an unholy land" fan fic...
Author's response
You know, this fic was finished far before that one. Besides, SIAUL is a fic where Harry travels to an alternative dimension and gains the skills of his evil counterpart. Plot wise the fics are completely different. There are a few similarities like the name 'Dark Knight' and the fact that Harry uses a sword. And you know, both of those are so terribly original... The author of SIAUL most certainly wasn't the first to use them.