Combat Expertise

You have learned to react quickly to your opponents attempts to better their position. You gain a special reaction you may use a number of times per round equal to half your proficiency bonus (rounded up) unless you spend a Ki point to use it again. When an opponent in your melee reach attempts to stand up from prone, you may use this special reaction to make an Attack of Opportunity. If the attack hits, that opponent must make a Dexterity saving throw (DC= Ki ability save DC). If the save fails, the opponent fails to stand up (movement is consumed as usual) and they cannot attempt to stand up from prone until the beginning of their next turn.