(Passive) Every time Hero kills an enemy, he/she enters grayplane for free.
For the Rogue ability to trigger, the killing blow must be dealt by a direct Hunter ability. Standard attacks count, ranged or melee, as do damage-dealing abilities like Flashcone and counter-attacks from Archery, but deaths resulting from indirect effects like Quellingmoss poison or Jumpjaw traps do not.
Rogue allows Hunters to position themselves more aggressively when they can guarantee a kill, as they will be safe from enemy attacks after entering grayplane. It also ensures that the Hunter's next attack will benefit from the armor-ignoring bonus of being hidden.