I'ma steal that name for the stalk ability but urn it into something else:

+ When looking at an enemy (the center of your crosshair aiming at the hitbox of the enemy) for every second you look at them, you + gain 10% movement speed; up to a maximum of +30% movement speed (sidenote, it's +1% speed for every tenth of a second, so it's gradual not immediate)
+ upon backstab user gains 50% charge on their watches
+ backstabs at maximum extra speed are quiet
+ damage increased by 1.7 times the base damage (don't know how to put that into a percent)
+ regular melee range (not backstab) increased by 15%
+ attacks with this weapon show no hit indicator on screen, neither does it give knockback, or a sound to the enemy that's attacked
+ for up to 1.5 second after backstab cloaking is twice as fast
- upon backstab user loses 10% extra speed immediately
- missed stabs (stabs meaning backstabs or butterknifes) remove extra speed, and decrease speed overall by 15%; this fades away after 2 seconds (stalking can remove the speed penalty at the same rate of the extra speed, so to fully make a comeback it would be 1.5 seconds, although it would be less than 1.5 as the speed penalty fades away slowly rather than instantly gaining speed back after the 2 seconds)
- attack speed decreased by 150% (1.2 seconds, the backstab is the same cooldown)
- 50% slower draw speed
- looking away from an enemy will decrease your additional movement speed at twice the rate (-20% extra speed per second, or 2% every tenth of a second)
-15 health
- upon backstab you will take 25% more damage for the next 5 seconds

Switching to a different weapon for more than a quarter second will remove the speed bonus

If this weapon is too bad still, I'd add a stack that after every kill you gain a +5% move speed (again up to the + 30%, where aiming at people doesn't do anything) for every kill. (this would be awesome at making people more convinced you're a scout. Oh, and trickstabs would be a lot easier)