D&Dtools1

While this spell is in effect, your melee attacks are more likely to strike a foe's vital areas.
Whenever you make a melee attack against a flanked foe or against a foe denied its Dexterity bonus, you deal an extra 1d6 points of damage, your weapon's threat range is doubled (as if under the effect of keen edge), and you gain a +4 insight bonus on rolls made to confirm critical hits.
The increased threat range granted by this spell doesn't stack with any other effect that increases your weapon's threat range.
Creatures immune to sneak attacks are immune to the extra damage dealt by your attacks.