Base Class: Fighter
Protect those close to you and tank the shit out of things. Make AC high and Protect the squishy ones in your party.
Two-Handed Defense
When you choose this subclass at 3rd level, you gain the ability to wield shields with great prowess. Your shields now have the heavy property and because of your mastery, it grants a +4 bonus to your AC. In addition, you can make attacks with shields. You receive the dual wielder feat but only works on your offhand weapon that is a shield. The damage die you use for your shield attack is 1d8 and it deals bludgeoning damage. At 10th level, this increases to 1d12.
Defensive Stance
At 3rd level, as an action you can take an all-out defensive stance when you're holding your shield. While you're in this stance, you have resistance to all damage except psychic and add your proficiency bonus to your AC.Your movement speed is halved when you are in a defensive stance. This stance lasts until the start of your next turn.
Provoke
Starting at 7th level, you may take an action to force all foes within 30ft to direct their single target attacks and actions at you until the start of your next turn. All targets affected must make a Wisdom saving throw against your Persuasion (charisma) check. You can use this a number of times equal to 1 + your Charisma modifier (minimum 1 time) per short or long rest.
Spell Blocker
At 10th level, you create a safe area in a 15ft cube behind you when you enter Defensive Stance. Allies within this cube add your shield's AC bonus to dexterity saving throws they make against spells or other harmful effects. If you or an ally are subjected to an effect that requires them to make a Dexterity saving throw to take only half damage, they take no damage if they succeed on the saving throw.
Undying Protector
At 15th level, you can no longer be critically hit, any attempt to knock you prone has disadvantage and the distance you can be shoved is halved. Also you can now hold your defensive stance for one minute.
Ultimate Shield
At 18th level you may enter a defensive stance as a bonus action instead of an action. When you enter a defensive stance you may add double your Strength modifier to your total AC.