Breaker Feat

You master the breaking of any creature within your grasp. You gain the following benefits:

Prerequisite: 4th level

• Ability Score Increase. Increase your Strength or Constitution by 1, to a maximum of 20.
• Punishing Blows. You have advantage on attack rolls against a creature you are grappling.
• Gain the Merciless Break special attack (below).

Merciless Break - Active Ability (Attack)

• When you take the Attack action on your turn, you can replace one of your attacks with inflicting debilitating injury on a creature you are grappling.
• The creature takes 2d6 + your Strength modifier bludgeoning damage and suffers an injuy for 1 minute. The number of d6 increases by one when your proficieny bonus increases (with the total number of d6 equaling your proficieny bonus). While suffering from this injuy, the creature subtracts d6 from attack rolls and ability checks, and must pass a DC 10 Constitution saving throw to successfully cast a spell.
• The target creature makes a Constitution saving throw at the end of each of their turns, ending the effect of the injuy on a success. A creature can spend their action treating their injury to automatically pass the saving throw.

Once you use this feature, you cannot use it again until you complete a short or long rest.
Source: KCLL, page 181

Active Martial Feat Save DC:

• Any time an active martial feat requires a saving throw, the DC is 8 + your Strength or Dexterity modifier (your choice) + your proficiency bonus.