TRAIT - Deadly
On a critical hit, the weapon adds a weapon damage die of the listed size. Roll this after doubling the weapon's damage. This increases to two dice if the weapon has a and three dice if the weapon has a . For instance, a Rapier with a deals 2d8 extra piercing damage on a critical hit. An ability that changes the size of the weapon's normal damage dice doesn't change the size of its deadly die.
Source: Core Rulebook pg. 282