How to make Nightcap a Stealth Character; Rework Ideas

I'd like a legendary upgrade that replaces fung fu.

something like... Shadow Daggers, it requires you to get close, where if you hit a zombie from the front it does maybe 65 damage and if you hit them from the back it doubles. And to make it something like roses thorn apart where you have to charge it to use it, either through stealth or vanquishes.  

Just an idea anyway..