Unaware (Quality)

A weapon with the 'unaware' quality may only be used against an opponent who is either helpless or who has no knowledge of your location before you attack. Only a stealth check greater than the opponent's perception check can grant the unaware status. Note that even invisibility doesn't make you silent, and foes get a perception check to notice you (albeit at -20 to the roll, per the standard invisibility rules). See Stealth for more information.