Swordmaster Feat

Prerequisite: Proficiency with longswords, shortswords, rapiers, or scimitars
Your swordplay leverages every advantage to build momentum, allowing you to strike or defend in new ways. When wielding a sword, you gain a special Swordplay d6 die each time you successfully attack a creature with it. You can have a maximum number of Swordplay dice equal to your proficieny bonus. You lose all accumulated dice if you stop wielding a sword or end your turn without making an attack with a sword.
You can spend your Swordplay dice on the following abilities:

• Sword Parry. As a reaction to being hit by a melee weapon attack, you can expend your Swordplay dice, rolling them and reducing the damage taken by the amount rolled. If the amount rolled exceeds the damage taken, the attacker takes damage equal to the difference.
• Sword Finisher. As a bonus action when you hit a creature with a melee weapon attack, you can expend your Swordplay dice, rolling them and dealing additional damage equal to the roll.

Source: KCLL, page 188