Way of the mantis warrior

The way of the mantis teaches its student to react, not enact. They focus on taking a deep stance and being ready to strike back at anyone who would attack them. 

Level 3: Mantis stance

You can use your bonus action and action to take the mantis stance until your next turn. While in the mantis stance your movement becomes 0 and you have advantage on any roll against being moved and anyone trying to move you in any way have disadvantage. While in the mantis stance, whenever you get attacked by an enemy within range you can expend 1 ki point and make an unarmed attack against that enemy. Your unarmed attack happens after the enemies attack. If the attack against you missed, you automatically hit your unarmed attack and if they rolled a 1 on their attack roll you crit them. You dont expend any action, bonus action or reaction with the unarmed attack.

If you are hit by an attack, roll a dexterity saving throw DC 12, if you fail your roll you are no longer in the mantis stance and no longer gain the benefits of being in the mantis stance. Your unarmed attack from mantis stance will still happen if you lose your mantis stance from the atack.

Level 6: Stance of a predator

When you are in the mantis stance, your gain bonus AC equal to your dexterity modifier.

When an enemy within you range make an attack against a creaute other then you, if you are in your mantis stance you may make a mantis stance unarmed attack, if you are not, you may use a reaction to make an unarmed attack against that creature. 

If an enemy attack a creature within 5 feet of you with a ranged attack, you may use your deflect missile reaction as if the attack was made against you.

Level 11: Force of a preying mantis

You have studied how a preying mantis wait and strik down their prey at the exact right moment with a strong force.

You no longer need to expend a ki point to make an unarmed attack while in your mantis stance. 

When you hit a creature when it isn't your turn, you can expend a ki point to strike them down. he target must make a Constitution saving throw. On a failed save, the target has the Prone condition and takes 10 bludgeoning damage. On a successful save, the target takes 10 bludgeoning damage.

Level 17: Predatory strike

You are the predator that catches your prey.

Attacks made while in the mantis stance is always a critical hit