How about this?

+Each Backstab increases your max cloak time by 2 seconds and cloaks 10% faster
(Max: 18 Seconds, 0.5 Second cloak/decloak time)
-30% Smaller Reach
-Cannot cloak as effectively for 2 seconds after a backstab(Ink will splatter on you, making your cloak revealing)

Originally posted by Constructor #1337420:

My stats are
! : on backstab, draws a marking on the ground 6 - 8 seconds after drawing it spawns a medium
health kit.
+ : 25% max health
- : 15% attack speed
- : 10% damage penalty

basicly a better Candy Cane, but for the Spy. Also, I dont think there should be a delay for the health pack to spawn, unless it's like 1 or 2 seconds. Also, I think it should be a small Med kit instead, medium sounds a bit over powered

Originally posted by Festive Pirate:

Originally posted by Constructor #1337420:

My stats are
! : on backstab, draws a marking on the ground 6 - 8 seconds after drawing it spawns a medium
health kit.
+ : 25% max health
- : 15% attack speed
- : 10% damage penalty

basicly a better Candy Cane, but for the Spy. Also, I dont think there should be a delay for the health pack to spawn, unless it's like 1 or 2 seconds. Also, I think it should be a small Med kit instead, medium sounds a bit over powered

That actually sounds much better, thank you for improving on my idea.

Originally posted by Constructor #1337420:

Originally posted by Festive Pirate:

basicly a better Candy Cane, but for the Spy. Also, I dont think there should be a delay for the health pack to spawn, unless it's like 1 or 2 seconds. Also, I think it should be a small Med kit instead, medium sounds a bit over powered

That actually sounds much better, thank you for improving on my idea.

Personally, I don't think this a set of good stats, the health boost and medkit heavily outweigh the debuffs, first off, the spy shouldn't even be using the knife for CQC unless its a backstab or trickstab, which negates the damage reduction and speed loss, the higher health would heavily outweigh this as it makes the spy last much longer than it really should, and the Health kit would make catching the spy even harder as that would allow the spy to extinguish himself and basically negate bleed or fire effects.

TL:DR In my opinion, these stats are majorly overpowered

This seems like a really cool idea

On backstab: enemy drops a small health kit
Silent Killer: No backstab noise
You can not disguise for 5 seconds after backstabing an enemy
There is a mark left on the ground where an enemy was backstabed and it stays there for 15 seconds.

+33% Swing Speed
Silent Killer
-66% Damage
Backstabing covers you in ink, effecting invis watches and leaving trail for 5 seconds