Versatile Weapon Master Feat

Prerequisite: Proficiency with a weapon that has the versatile property
You are an expert on the fluid use of versatile weapons, changing stances to fit any situation. When you draw a versatile weapon you are proficient with, or start your turn while wielding one, you can enter a special stance as long as you aren’t wielding other weapons or a shield.
This lasts until the start of your next turn, drop or sheath your weapon, or equip a shield. Each stance provides modifiers to your melee weapon attacks made with the versatile weapon while in that stance:

• Neutral Stance: You add +1 to your attack rolls, damage rolls, and armor class.
• Power Stance: You don’t add your Proficieny bonus to attack rolls, but add twice your Proficieny bonus to your damage rolls.
• Accuracy Stance: You add twice your Proficieny bonus to attack rolls, but don’t add your Ability modifier to damage rolls.
• Defensive Stance: You don’t add your Proficieny bonus to attack rolls, but if you hit at least one attack against a hostile creature, you can add your proficiency bonus to your AC until the start of your next turn.

Source: KCLL, page 189