The tooltip says it does a guaranteed, bigger damage when hitting enemies while stealthed, which works just fine.
The problem here, it doesn't work with Nightblade's Invisible skill.
Before anyone mentions that it's for STEALTH only, as in first hit, the exact same wording can be found in the skill Veiled Strike and it does proc out of invisibility.
If the skill weren't meant to work with invisibility, they should've said so and I would've picked the other morph.