'Silent but deadly' is a combo that pops up when you meet the requirements below. If you're on Steam, you get an achievement alongside this.
SILENT BUT DEADLY COMBOPeople are complaining about an awful smell that appears to suddenly appear out of nowhere!
Guide
Evolve Corrupted Air, Night Wraith and Dark Infiltration and then wait for the pop-up, with the title of 'Silent but deadly combo'.
Steam achievement.