Assassination knife

As an action you shroud this golden knife in shadows. Make a melee weapon or spell attack. On a hit the shadows flow into the wounds, dealing 10d6 necrotic damage on top of the regular weapon damage (this spell stacks with Sneak attack if you have it). Then the spell ends. 

At higher levels. you can cast this spell at higher levels. add 1d6 for every level above 3rd.