Shield Maiden

The Shield Maiden is a natural born warrior. Her furiosity in battle in matched only by her bravery on the field. Those that choose this path are said to be blessed by the Valkyries themselves.

This subclass is made by Asa Wheatley, and can originally be found in the Uncaged books. I translated it to Dndbeyond for simple usage and accessibility. 

Shield Bash

Starting at 3rd level, you have honed your skills with a shield, enough to be able to use it in battle for both defence and offence. As a bonus action you can make a Shield Bash attack. Dealing 1d4 bludgeoning damage on a hit. You are profcient in these attacks and they are based of strength. When you hit a creature with your Shield Bash attack they must immediately make a Strength saving throw (DC 8 + Strength Modifer + Proficiency Bonus) or be knocked prone.
The damage dealt by your Shield Bash attack increases when you reach certain levels in this class, increasing to 2d4 at 10th level, 3d4 at 15th level and 4d4 at 18th level.

Unflinching Bravery

Starting at 3rd level, you are impossible to shake and your skills in battle make you one to be feared. You gain proficiency in the intimidation skill if you do not already have it and you are now immune to the frightened condition.

Fury of the Valkyries

At 7th level, as a bonus action a Shield Maiden can call upon the powers of the Valkyries and imbue herself with their fury. For 1 minute your bonus to all damage rolls based of Strength is doubled. When the Fury fades the Shield Maiden is left physically drained and they immediately suffer 1 point of exhaustion. Once you use this feature, you must finish a short or long rest before you can use it again.

War Cry

At 10th level, when an enemy creature you can see targets you with an attack you can use your reaction to let out a furious war cry. The creature is taken off-guard and their turn immediately ends before they can complete the attack. You can then immediately take your turn despite where you fall in the initiative order. Your placement in initiative goes back to normal in the following round of combat. Once you have used this feature, you can’t use it again until you finish a short or long rest.

Overwhelming Strikes

At 15th level, your mastery of battle allows you to overwhelm your enemy. If you hit a single creature with three or more melee attacks on your turn they
must make a Wisdom saving throw (DC 8 + Strength Modifier + Proficiency Bonus). On a failure, the target is overwhelmed by your attacks, they have disadvantage on all attacks made against you and you have advantage on all attacks made against them until the end of your next turn. If they attempt to move out of your combat range before this effect has faded they can only do so if they first take the disengage action.

You can use this feature twice, and you regain all expended uses of it when you fnish a short or long rest.

Perpetual Defence

Starting at 18th level, your skills with your weapons and shield are unmatched, using them for both offence and defence simultaneously. You can no longer be flanked and all attacks of opportunity against you are made at disadvantage.