The 5th bonus from this set gives you 369 wp and sp damage when you leave Sneak or invisibility *while in combat* for 15 seconds.

I can proc it out of combat and I guess its not just me with that bug.

I can just enter invisibility anywhere not in combat and I get the stygian buff and my wp/sp damage goes up.

So as a Nightblade Ganker for example you can get the buff before going into combat.