Mist form isn't invisibility, so it doesn't proc stygian.
This set is trash again, so give it a miss until it's inevitable rework. Even if you wear this on a nightblade and cast invisibility perfectly for 100% uptime, you would still have done more damage if you were in julianos casting one more DoT every 15 seconds.
#1 tip (Re)check your graphics settings periodically - especially resolution.
The mist doesn't, but the 3-sec-run invisibility you get on stage 4 does. So if you run up to trash mobs, you can nuke them pretty quick with both damage buffs. Great for solo play, vampire in general, especially if you don't play "serious" content.
"The paper is balanced, but nerf the rock!"
- The scissors