So since the current state of this ability is just awful, I might have a fun idea for it. Remove its damage component, keep the silence, and also make the target immune to all damage for the duration of the silence (maybe freeze their regens too).
I feel like this would be ok for the class because nightblade already has incredible burst potential without incap, it just takes a few light attacks before it's ready (which would likely also be a heavy nerf to ganking, oh noes).
Imbue weapon, relentless focus, light attack, surprise attack, can all go off at the same time. That's still huge burst without an ultimate.
Why do this?
Give nightblade an important role in group vs group PvP. The nightblade silences the healer for 3 seconds which can be timed with a burst from the rest of the group. If the target group has no defenses/heals without the healer, then it's a wipe. If the group survives long enough for the healer to become unsilenced, then the big heals come in again. Note that the healer must be immune to damage during the silence otherwise the other group would just focus the healer every time for an easy wipe. It allows for counterplay if the incap target is immune to damage.
Give nightblade an interesting 1vX mechanic. They can pick an enemy to silence in order to alleviate pressure off of themselves, which gives them a window to finish a kill on a different player.