When making an attack roll, if the player (or DM) rolls a 'Natural' number that falls within the critical threat range of the weapon being used, a 'Critical Threat' is observed. (This includes magic or feats that increase the critical threat range of a weapon).
The person in question then rolls a d20 again, adding any attack bonuses. If he rolls what would normally be a hit against the target, he has scored a critical hit. He then rolls damage and multiplies the damage rolled by the Critical multiplier of the weapon.
If the roll would not be a hit, then the attack does normal damage.
If, on the original attack roll, you roll a ‘Natural’
20 and on the 'Confirmation' roll, you roll a 'Natural' 20, you have scored an
'Instant Death' effect.
Sample: Dorius attacks
a Gnoll with an AC of 20, using his Keen Longsword. His attack bonus with the weapon is 7. He rolls
an 20 on the d20, which falls into the critical threat range of his enhanced
He then rolls the attack die again, depending on
the result, he gets:
Rolls a 12 or less (Which is not a hit): He does
Rolls a 13 through 19: He gets a critical
Rolls a 20: He get an 'Instant Death' effect,
unless his target has action points to spend, it dies instantly.