Stone Monkey (5e Feat)
From D&D Wiki
When grappling a creature, you may replace one or more of you attacks with attemps to cripple the grappled creature. the opponent makes a Constitution saving throw against 8+proficiency+Dexterity or Strength modifier(whichever is higher). On a failed save you dislocate a limb of the creature. All checks made using this limb have disadvantage, attack rolls using a dislocated arm have disadvantage. Movement speed is halved if the limb was a leg, if all a creature's legs are dislocated the creature's speed becomes 0. Dislocated limbs remain dislocated until the creature is healed in any way by magic or another creature performs a DC 15 Medicine check.