Critical hit multiplier

The critical hit multiplier is the number by which the base damage is multiplied upon a confirmed critical hit. Not to be confused with critical threat - which refers to how likely you are to land a critical hit. It also affects various damage effects coming from specific weapon enhancements;
 * Bursting effects
 * Elemental Burst
 * Holy Burst, Unholy Burst
 * Good Burst, Evil Burst
 * Maiming

Ways to increase Critical hit multiplier

 * Monks that choose Master of Stone or Grandmaster of Stone stances gain +1 critical multiplier to monk weapons and unarmed attacks on a roll of 19-20. Master/Grandmaster Mountain stance multiplier increases stack when combined with The Trembling Earth Earth-Earth-Earth +2 critical multiplier finisher for a x5 multiplier when unarmed, on a roll of 19 or 20.1


 * Barbarian Frenzied Berserker II provides a +1 increase to critical multiplier with any weapon, on a roll of 19-20 when raged. Barbarian Frenzied Berserker III provides an additional +1 increase to critical multiplier with any weapon, on a roll of 19-20 when raged. It's sub-enhancement - Death Frenzy provides yet another +1 increase to critical multiplier, on a roll of 19-20 while frenzied. All of these stack for a total of +3 multiplier when both raged and death frenzied. This would bring a Frenzy Berserker wielding a heavy picks critical multiplier up to a impressive x7 on rolls of 19-20.


 * Paladin Exalted Smite II enhancement increases the critical multiplier when smiting by +1, and Exalted Smite III enhancement by another +1 (for a total of +2 to critical multiplier). This rise functions on any attack roll that is a critical threat (unlike the Monk and Barbarian case). Exalted Smite I and IV also increase the weapons threat range by +1 each.


 * Paladin Divine Sacrifice I enhancement increases the critical multiplier for the next attack by +1 against evil opponents.

1Citation on official forums