Like Kaled Said, nice concept. Background needs some work, as do the stats, but then, so does everyones first character.
The way I like to do a model is come up with a concept, find a model and bitz that fits the concept, build the model, and then develop the character. Everyone has a different style (Marco Skull is so devoted to getting exactly as he likes it he learned to scult and makes his own from scratch now

).
With that, a model suitable for being a black shadowy death assasin, and using a handbow, shouldn't be too hard to find. My advice is rummage through your bitz box and see what you have on hand, use the Inquisitor modelling resources thread (
Found Here to look through various manufacturer's to see what's available as a base figure. Eoliths Dracula might work quite well (and has just been updated), hang some grenades on his belt, hand swap for whatever crossbow you can conjure up, and either greenstuff or headswap to give him a cloaked head. Of course, that's just one idea, there's bound to be dozens if not hundreds of other base models, and thousands of ways to do it.
When it comes to the rules, I'd probably drop true grit and lightening reflexes, and tone down the strength and toughness a little. And use Stats that are not just multiples of 5, it's a d100 scale for a reason

.
Why does his knife have reach 2 when blademaster makes it reach 4 anyway? Why not have a different skill than blademaster, and use a short sword? How Many Quarrels does he have? And why not coat all his weapons with the poison (at least the throwing knives).
The Medipak doesn't seem very characterful to me for an assassin. What about some does of an antidote to the hallucinogen he uses instead? Might be useful incase he or an ally gets dosed by accident.
When it comes to background, is he going to be connected with a warband? If you are intending to take on whole other warbands with one figure, you can bump up the stats and gear quite a bit, and I'd still say the odds are against you. Like Darksinger says, if he's on his lonesome, then he's if anything underpowered.
If he is connected to a warband of an inquisitor, or such like authority figure, background can be a fun if slightly formulaic description of his cult, his recruitment by the authority figure, and some of his recent escapades.
A devout cult that believes to not worship the emperor is madness, and so the enemies of the emperor should be killed by madness for example, could be a fitting cult for his origins.