Back to Back Fighting

 You've gained the ability to fight back to back with another person, defending them and allowing them to defend you.  You gain the following benefits.

•   When an ally is within 5 feet of you while you are holding a melee weapon, you may use a bonus action to impose disadvantage on all attacks against that ally until the start of your next turn.  This effect stops if you or your ally move away more than 5 feet from each other.

•   While you are defending an ally in this way and you are hit with an attack while that ally is holding a melee weapon with at least as much range as yours, that ally may use their reaction to add their proficiency bonus to your armor class against that attack.

•   When an attacker misses an ally that you are defending, you gain advantage on the next attack roll against that creature until the end of your next turn.