Source: Player's Handbook
Master Sophisticated Battle Maneuvers
Battle Masters are students of the art of battle, learning martial techniques passed down through generations. The most accomplished Battle Masters are well-rounded figures who combine their carefully honed combat skills with academic study in the fields of history, theory, and the arts.
Level 3: Combat Superiority
Your experience on the battlefield has refined your fighting techniques. You learn maneuvers that are fueled by special dice called Superiority Dice.
Maneuvers. You learn three maneuvers of your choice from the Maneuver Options. Many maneuvers enhance an attack in some way. You can use only one maneuver per attack.
You learn two additional maneuvers of your choice when you reach Fighter levels 7, 10, and 15. Each time you learn new maneuvers, you can also replace one maneuver you know with a different one.
Superiority Dice. You have four Superiority Dice, which are d8s. A Superiority Die is expended when you use it. You regain all expended Superiority Dice when you finish a Short or Long Rest.
You gain an additional Superiority Die when you reach Fighter levels 7 (five dice total) and 15 (six dice total).
Saving Throws. If a maneuver requires a saving throw, the DC equals 8 plus your Strength or Dexterity modifier (your choice) and Proficiency Bonus.
Level 3: Student of War
You gain proficiency with one type of Artisan’s Tools of your choice, and you gain proficiency in one skill of your choice from the skills available to Fighters at level 1.
Level 7: Know Your Enemy
As a Bonus Action, you can discern certain strengths and weaknesses of a creature you can see within 30 feet of yourself; you know whether that creature has any Immunities, Resistances, or Vulnerabilities, and if the creature has any, you know what they are.
Once you use this feature, you can’t do so again until you finish a Long Rest. You can also restore a use of the feature by expending one Superiority Die (no action required).
Level 10: Improved Combat Superiority
Your Superiority Die becomes a d10.
Level 15: Relentless
Once per turn, when you use a maneuver, you can roll 1d8 and use the number rolled instead of expending a Superiority Die.
Level 18: Ultimate Combat Superiority
Your Superiority Die becomes a d12.