The Way of the Unbreaking is a path of fortitude, will, and the ignorance of pain itself.  To a monk of the Way of the Unbreaking, it's not about how hard you can hit, it's about how hard you can get hit and keep moving forward.  These monks become masters of their own constitution and mortality.  If invulnerability is their aim, they may near achieve it.

Fortitude

Starting when you choose this tradition at 3rd level, you have hardened yourself to become tougher.  You gain a bonus to your AC equal to your Constitution modifier.

Indomitable

At 6th level, your body and mind have grown stronger.  Anytime you take damage, you may use your reaction to resist the number equal to your Constitution modifier of that damage.  In addition, you may spend 2 ki points to resist an additional (1d6 + half monk level) damage.

Death's Door

Beginning at 11th level, your skill in the Way of the Unreaking has reached the point where death itself flinches at your presence and you feel no fear in its. You have advantage on death saving throws and gain immunity to the frightened condition.

Unkillable

At 17th level, you have mastered the Way of the Unbreaking.  Once per long rest, when you fall to 0 hit points, you instead fall to 1 hit point.