Intensive/Focused Combat

 After gaining skill in martial combat, you gain the ability to force an enemy to focus only on you.

You and your opponent are locked in melee combat, neither of one can deal damage to each other and can't target others. Whoever disenages first to attack or flee, the other has opportunity to attack with advantage. 

At level 15, you can engage two enemies at once with this. However, if you change targets or run, both threats get attack of opportunity. If one of them disengages, you can opportunity attack them, but you lose the effect on the other opponent, who takes turns normally after that.

At level 20, you can engage 3 opponents. 

You can only disengage on your turn, or as a reaction, such as an enemy not dueling with you giving you opportunity attack. 

If you are damaged by another enemy, make a DC Con saving throw. If the damage is 10 or less the DC is 10. 11 to 25 is 14. 26+ is 17.