So, um.
This mod changes the perk Shadow Warrior to add invisibility when you crouch in addition to the current perk attributes.
It will add an active effect (Shadowcloak of Nocturnal, b/c I used that as the basis) to your magic menu, but it will only take effect when crouching.

EDIT: I fixed it for me in the latest (3.5) version, so make sure to get that if it wasn't working for you before. When you install this version, press your 'power' button first (when you first load the save). Make sure to have any powers unequipped before pressing the button though. The invis on crouch should take effect after pressing. You might have to press it again after using other powers and unequipping them though. Have fun!

Install:
Drop .esp to Data
Uninstall:
Delete .esp