When wielding slashing weapons the whirlwind attack doesn't take into account weapon material or enchantment. So for example creatures immune to non-magical weapons like shadows take zero damage from the extra whirlwind even when attacking with a +4 weapon.Northlander Hewing
Northlander Hewing
Type of Feat: General / Fighter [Style]
Prerequisite: Strength 13, BAB +6
Use: Selected (mode)
Northlander hewing is a style of melee combat made famous by the Northern barbarian tribes, who are renowned for the savage strength they can put behind each blow. You limit yourself to 2/3 of your attacks (rounded to nearest integer). You add your strength modifier as extra physical damage. Due to the strength you must put behind your blows you suffer a dodge AC penalty of 2.
Randomly triggered ability depending on your weapon type added: Ground slam for bludgeoning (Knockdown is HD/2 + 2d8 DC), Bleeding for piercing (STR mod bleed damage for 2 rounds), Whirlwind for slashing weapons (Dam = 2d10 + STR mod).
Unsure if its a bug, but would also make sense if the whirlwind applied the wielded weapon's elemental/damage bonuses and/or vampiric regeneration effects.