Specifics: Whenever the character makes a successful attack against an opponent who is flat-footed, cannot see them, or who is in combat with someone else, the character's blow delivers extra damage. This extra damage is +1d6 at fourth level and an additional +1d6 every three levels thereafter. This extra damage is not multiplied in the case of a critical hit. Use: automatic. Monsters of the construct and undead types are immune to sneak attacks, as are any creatures that are immune to critical hits.