The Battlemaster Tunic is a uncommon Torso Armor. It can only be obtained by killing and looting a Pirate Battlemaster. Although considered uncommon, the whole set tend to be rather common when encountering many Pirate Battlemasters.
Source
Trivia
Set Bonus: (3) Sprint for less stamina
- Battlemaster Tunic
- Battlemaster Pants
- Battlemaster Boots