Unlike most monks whose main fighting styles involve dodging or parrying their adversaries, those who walk the Way of the Unbreakable Body are masters of endurance. Rather than avoiding the blows of their enemies, they learn to absorb them. They channel that force into their body and redirect it into potent ki strikes, turning their enemies' attacks back upon them. Monks of the Unbreakable Body understand the virtues of fortitude...and that should they come close to death, it is then that they are pushed to their limits, their maximum potential becoming unlocked.

Honed Endurance

Starting at 3rd level when you choose this tradition, your maximum hit points increase by an amount equal to your monk level, and you gain proficiency in Constitution saving throws.

Fortitude

Starting at 3rd level when you take this tradition, you gain the ability to absorb blows from your foes and channel them into potent strikes. You gain a pool of points called your Fortitude Pool. Your pool starts at 0 and whenever you take damage from another creature, you add an amount of points to it equal to the damage you take. The pool's maximum total is equal to 10 times your monk level.  These points remain in your pool until you take a long rest. If you are reduced to 0 hit points, you loose half the currently accumulated points in your Fortitude Pool.

You may use the points accumulated from your Fortitude pool to perform the following abilities.

Rebounding Force. When an attack with an unarmed strike or monk weapon hits, you may spend 1 ki point and points from your Fortitude Pool in increments of 5. For every 5 points expend from your Fortitude Pool, your attack deals 1 extra damage. You expend up to a maximum of 25 points from your Fortitude Pool, granting a maximum bonus of 5 damage to a single attack.

Pained Ki. When you use a class feature requiring ki points that's not a part of the Fortitude class feature, you may expend 10 points from your Fortitude Pool in place of 1 ki point. You can do this for multiple ki points with the same ability.

Soothe. As a bonus action, you can spend 1 ki point and as many points from your Fortitude Pool as you like. You restore a number of hit points equal to half the points you expended from your Fortitude Pool.

Author's Note: As of the time of writing, there is no way to add custom resources to subclasses on the DnD Beyond character sheet. To keep track of your Fortitude Pool, use the Notes section of character sheet or another manual method at your disposal, such as a word processor on your computer or good old pen and paper. If a feature that allows me to add a resource counter to subclasses is implemented, I will endeavour to update as soon as possible.

Brace

Starting at 6th level, you learn to steady yourself to absorb blows. As a bonus action, you may spend 1 ki point to gain resistance to all damage until the start of your next turn. Any damage you take for this duration adds twice the number of points to your Fortitude Pool for each point of damage you take.

Unflinching Counter

Starting at 11th level, you learn to counter any attack made against you. Whenever a creature within your melee attack range deals damage to you with a weapon attack, you may use a reaction to make a melee weapon attack against that creature with advantage.

In addition, you gain two extra reactions you can use each round. You may only use these reactions to use this feature.

Absolute Limit

Starting at 17th level, you are pushed to your physical limits while you are on the cusp of death. Your Brace ability is permanently active as long as you are below one quarter of your maximum hit points.