Hello mate! Welcome to the forums!
Having played a Nightblade a bit I have a pretty good feel of the class, and I thin your build idea there has some promise. IF you can manage to stay in stealth and not be seen, you could be absolutely deadly! But, as stealth centered as it is, I do see some issues when attacking castles or other large-scale fights, as that many enemies around would make stealthy kills very difficult. Thus I would say have a good secondary skill-bar handy for such situations, perhaps a mix of dual wielding and siphoning abilities for survivability