Aerial Guardian (5e Feat)
From D&D Wiki
Prerequisites: Strength 15, Constitution 18
You've mastered a defensive style of fighting, perfect for aerial combat. When you are in flight and wielding a melee weapon with which you are proficient and is classified as Heavy, you gain the following abilities:
On your turn, as a Bonus Action, you may enter a defensive stance. While you're in this stance you gain +2 to your AC and your flying speed is halved.
By spending an additional 5ft per 5ft of movement you may move without provoking opportunity attacks.
When a creature within 30ft of you hits an ally with an Attack, you can use your reaction to change into energy and shoot towards an unoccupied space within five feet of the attacking creature before changing back into your physical form. As part of the reaction, you can take the damage meant for your ally. You can use this feature a number of times equal to your Constitution Modifier. You regain expended uses of this feature when you finish a long rest.
When you hit a target outside of your turn, you may add your proficiency bonus to the attack and damage roll of that attack.
Being in defensive stance is not required to use other parts of the feat.