"Thunder and Fang (Combat)
You have mastered the ancient Thunder and Fang fighting style, allowing you to fight with increased effectiveness when wielding an earth breaker and klar.
Prerequisite: Str 15, Two-Weapon Fighting, Weapon Focus (earth breaker), Weapon Focus (klar)
Benefit: You can use an earth breaker as though it were a one-handed weapon. When using an earth breaker in one hand and a klar in your off hand, you retain the shield bonus your klar grants to your Armor Class even when you use it to attack. Treat your klar as a light weapon for the purposes of determining your two-weapon fighting penalty.
Normal: An earth breaker is a two-handed weapon, preventing the use of a klar in one hand without imposing penalties for using the earth breaker one-handed. A klar can be used either as a one-handed weapon or a shield; it does not grant a bonus to AC during rounds in which it is used as a weapon."
Where can I find what the penalty is for using a two-handed weapon one-handed? Is it just an error?
There are ways, such as a Jotengrip barbarian to use a two-handed weapon one-handed. However, without special abilities from an archetype, feat, or the like, you can not wield a two-handed handed weapon in one hand.