Starting at 18th level, you can use your reaction to deflect a blow when you are hit by a melee weapon attack. When you do so, the damage you take from the attack is reduced by 1d10 + your Dexterity modifier + your monk level.
If you reduce the damage to 0, you can catch the weapon disarm the attacker. Instead of catching a weapon in this way, you can spend 1 ki point to turn the attack against the attacker, as part of the same reaction. You make an unarmed strike against the assailant, dealing normal damage, plus the damage you reduced the strike by.
As a bonus action, you spend 5 Ki points and lower all of your physical defenses. Your AC drops to 10, and you automatically fail Strength, Dexterity, and Constitution saving throws to resist damage. Keep track of how many hit points you lose while in this state. This state ends when you make an attack or deal damage to a creature or after one minute. While in this state, you may use a reaction after taking damage to strike a creature with a melee weapon attack at advantage. The next time you deal damage after entering this state, you deal additional damage equal to two times the number of hit points you lost while in this state.