Battle Dancer

If you Incapacitate an enemy by reducing it to 0hp with a melee, you can use a Bonus Action to gain a free melee attack using the same weapon on any other creature with 5 ft. This does not provoke an attack of opportunity. However, any further attacks do grant an attack of opportunity. If you get a killing blow during that Bonus Action, you can gain 1 additional Bonus Action that you must use to make a melee attack with the same weapon against a new creature within 5 feet of the one just killed. This can repeat a maximum number of times equal to your Proficiency bonus. This counts as part of your maximum movement for the round. If you are out of movement, you can't use this feat.

A level 5 Fighter would have a Proficiency bonus of +3. This would mean that after downing a foe, he uses his Bonus Action to attack a 2nd enemy 5ft away (this uses one of the three maximum bonus actions) . The Fighter kills the 2nd and uses the Bonus Action to attack a 3rd enemy (use +2). The 3rd dies and he reaches the maximum number +3, equal to his Proficiency bonus. Even if he kills the 4th, he can't do anything else this rd. And he's used 20 ft of movement for the rd.

This can also incur a morale check by the enemy at the DM's discretion, seeing as how 1 dude just mowed down 4 people like he was taking a walk through a park.