Grand Master
Prerequisite
Master in the chosen weapon, Intelligence 18 or higher
Unlike most feats, this one is purchased with an investment of downtime and money. The cost of this feat is 12 weeks of time and 12,000 GP. In addition to the prerequisite cost of 14 weeks of time and 10,500 GP.
This skill level grants enhanced skill in the use of a specific weapon. It allows the character to utilize greater damage and attack capacities. In your hands the chosen weapon is more accurate and lethal than typical and other weapons you wield.
You add four to attack rolls with the chosen weapon.
You decrease the chosen weapon's damage dice size by one (from the base weapon), add eight plus twice the maximum original die roll to the roll.
If your chosen weapon is a melee weapon, you increase your AC by 2 while the chosen weapon is being wielded. If your chosen weapon is a ranged weapon, increase it's range by 100%.
Starting at this level of training you start gaining a set of abilities that are fueled by special dice called superiority dice. You have three superiority die, if you have this feature the die is an additional three dice otherwise it is d6. If you already are able to perform the listed maneuver from another source, your superiority dice grows one size larger when performing the listed maneuver with this weapon. A superiority die is expended when you use it. You regain all of your expended superiority dice when you finish a short or long rest. You can use only one maneuver per attack.
By expending a superiority die while wielding the chosen weapon, you may any of the below actions:
Dodge. You may take the dodge action as a bonus action.
Disarming Attack. When you hit a creature with a weapon attack, you can expend one superiority die to attempt to disarm the target, forcing it to drop one item of your choice that it's holding, You add the superiority die to the attack's damage roll, and the target must make a Strength saving throw, On a failed save, it drops the object you choose, The object lands at its feet.
Parry. When another creature damages you with a melee attack, you can use your reaction and expend one superiority die to reduce the damage by the number you roll on your superiority die + your Dexterity modifier.
Riposte (If the chosen weapon does not have the reach property). When a creature misses you with a melee attack, you can use your reaction and expend one superiority die to make a melee weapon attack against the creature, ]f you hit, you add the superiority die to the attack's damage roll.
Trip Attack (If the chosen weapon has the reach property). When you hit a creature with a weapon attack, you can expend one superiority die to attempt to knock the target down, You add the superiority die to the attack's damage roll, and if the target is Large or smaller, it must make a Strength saving throw, On a failed save, you knock the target prone.
This feat replaces it's prerequisite feat.
Source: Master Player's Book, page 15.