I was inspired to make this subclass from both cavalier and tunnel fighter feat. I found that tunnel fighter was too powerful yet fun but cavalier was too high of a level to really get to experience it. I decided to dumb it down and see if its balances out the op tunnel fighter feat.

Defender Stance

Starting at lvl3 You became adapt at reacting to danger. As a bonus action you can enter a Defensive Stance granting you a number of interception points equal your proficiency bonus to use during the round. During combat, you can use a special reaction to do attacks of opportunities once on each creature turn except your own. You cannot use it on the same turn that you use your normal reaction.

Challenging roar

Starting at lvl3 you can call a challenge out to enemies up to your proficiency bonus within 30 ft of you. The enemies will have disadvantage with attacks against any player other than the fighter.

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.

Defend the Weak

At 7 level you learned to not only react to danger incoming to you but also allies. When you see an ally within 10 ft of you take a hit from an attack you can use your reaction to spend 10ft of your movement to charge over and take the hit in the allies’ place. You lose this movement for your next turn.

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

None Shall Pass

At 10 level, you want to defend your friends from all danger that you learned to block your enemies from maneuvering within your reach. Creatures that move 5 ft or more with in your reach will provoke an attack of opportunity, and if you hit a creature with an attack of opportunity that creatures speed is reduced to 0 until the end of the current turn.

Defenders Might

Starting at 15 level, your resolve has become as strong as Adamantine. You gain advantage on saving throws against spells.

Defender Stance Mastery

Starting at level 18, you mastered the ability to defend against the on slot of foes. While in Defensive Stance, you have advantage on attacks of opportunity.