Protector

A fighter master of the shield and a defender to the weak.

Taunt

As a bonus action you can provoke a creature within 60ft. to attack you. When the creature starts it's turn it must make a WIS save or suffer disadvantage on any attack that is not directed at you. The creature can repeat the save at the start of each of it's turns. You can use this feature a number of times equal to your proficiency bonus.

Shield Master

You gain 1 bonus AC if with shield and you can chose one ability to perform with a shield. You can perform your abilities once at lvl 7, two times at lvl 10 and three times at lvl 15.These abilities reset after a short rest.

(1 bonus action) Shield Stun - Make an attack with your shield on a hit slam your enemy with your shield and stun them. The target must succeed on a Constitution saving throw or be stunned until the end of it's next turn.

(1 bonus action) Shield Slam - Make an attack with your shield on a hit the creature must succeed on a STR saving throw or take 1d6 damage and is pushed 10ft. Targets that concentrate on spells have disadvantage on concentration checks until the start of their turn. If it succeeds it takes half damage and is not pushed nor gains disadvantage on concentration. 

(1 bonus action) Shield Defense - Use your shield to guard yourself from any attack. You can use this ability to make the enemies attack you with disadvantage until the end of your next turn.

Ultimate defense

You are a master of defensive combat as an action you can use your ultimate defense technique to gain resistance to all physical attacks until the start of your next turn. This ability resets after a long rest. Can be used a number of times equal to your proficiency bonus.

Defender

Every ally within 5ft of you gains +1AC

Final fortress

As a master of defensive combat you don't give up easy and the closer you are to death the harder you are to kill. When determining the threshold round down the hp difference.

2/3hp +1 AC

1/2hp +2 AC

1/3hp +3 AC