Shielder

You have mastered the art of the shield, able to perform feats that any other cannot. You have honed your ability to protect and deter with the shield to the extent that any other weapon hinders you... not that you need any other. Your shield is your weapon, and the greatest offense begins with a good defense.

Shield Expert

Starting at 3rd level, your mastery in the shield allows you to use it as a weapon. For you, while you are holding no other weapons other than shields, a shield counts as a martial weapon you are proficient in. It deals 1d6 bludgeoning damage on a hit and has the light property.

You have honed your body to be resilient against damage. Your hit point maximum increases by 3 and increases by 1 again whenever you gain a level in this class.

I'll Protect You!

Starting at 3rd level, you can defend your allies effectively. As a bonus action, you can prepare your shield and designate a creature. Until the start of your next turn, while you are within 5 feet of the designated creature and you are holding a shield and no other weapon, all attacks directed at that creature are directed at you instead.

You can use this feature a number of times equal to your Constitution modifier (minimum of once), and you regain all expended uses of it when you finish a long rest.

Dual Shield

At 7th level, you learn to enhance your defenses even further. When you wield two or more shields, you can gain the benefits of +2 AC on top of AC you gain from wielding the first shield (you can add the item Shielder's Dual Shield to reflect this ability) and you can use your shields to grapple or shove enemies. In addition, if you are using a magical shield to attack, your attacks count as magical for the purpose of overcoming resistance and immunity. 

Wall of Snowflakes

At 10th level, you are able to be a wall that perfectly defends your allies. When an area of effect spell or ability affects an ally within 5 feet of you and you are holding a shield, you can use your reaction to move beside your ally and stand between the point of origin and that ally. If you do, you are considered total cover for that ally. Creatures in a 15-feet cone behind you (including that ally) do not suffer the effects of the spell or ability. You make saving throws as normal. 

Once you use this feature, you can’t use it again until you finish a short rest.

Unbreakable Wall

Starting at 15th level, your protection has surpassed mortal boundaries, allowing you to defend an ally so perfectly against an enemy that nothing that enemy does can go past your defenses.

As an action, you can choose a creature you can see within 30 feet of you as your protection target (including yourself). Until the start of your next turn or until your concentration ends (as if concentrating on a spell), as long as you are wielding no other weapons other than shields, the protection target is considered immune to all damage.

Walking Castle

Starting at 18th level, your extraordinary ability to defend has taught your body how to defend itself instinctively. You have resistance to all bludgeoning, piercing and slashing damage from any source as long as you are wielding a shield, wielding holding no other weapons other than shields and you are not incapacitated.