I found this story a couple weeks ago and i find myself hanging onto every chapter and egearlly awaiting the next. For a FanFic this is as close as i gets to black library Quality.
Child of the Emperor
http://www.fanfiction.net/s/8666609/1/
"Do the traitor legions hold any regrets? This is the prologue to a much larger tale, which is Mrated. If enough people like it, I'll start putting the additional chapters online. Be warned; the full version is a long story currently at 134k wordcount with another 60k to write. Still, I'd appreciate any comments."