Monks of the Way of the Ninja use their ki in combat to give them an advantage in combat, from disarming foes to blinding them. They have learned to use their ki both in an offensive manner and a defensive manner, forming weapons out of pure ki. They have also learned to control their minds helping them further.

Ninja Training

Starting at level 3 through your training you have learned how to sneak around better than before, giving you an advantage on dexterity (stealth) checks. you've also learned to clear your mind, giving you an advantage on saving throws when being charmed or confused. You also cant be put to sleep or be frightened. 

KiJutsu

You have learned to fight in a new way. Starting at level 3, as a bonus action, you can create a staff pointed at both ends, this weapon has the same properties as a quarterstaff, and a spear. As you learn and progress in this path (3rd, 6th, 11th, and 17th levels) you will learn different ways to attack and defend yourself with your ki weapon (quarterspear). You can add 2 to your AC when you have it out, due to the fact that you've learned to defend yourself with the quarterspear (At the end of your turn, if your quarterspear is not in your hands, it disappears).

Disarming Strike

At level 6th, you've now learned to use your quarterspear to disarm people. As a bonus action, you can spend 1 ki point to hit another creature's arm and send a little shockwave through that arm. That creature must make a Strength saving throw (if the creature is holding their weapon with two hands they have an advantage on the saving throw). On a failed save, that creature's weapon gets knocked out of their hand and thrown 5 feet away, and they take 2 damage (on a successful one they still take the damage). The saving throw DC is your class level + your Wisdom modifier + your proficiency bonus (At 11th level, this attack does 5 damage, and knocks their weapon 10 feet away).

Ki Flash

Starting at 6th level, you've learned to form a ball of pure ki in your hand and can launch it at any one creature within 10 feet of you as a bonus action (make an attack roll for that creature). That creature takes radiant damage equal to your martial arts die and is now blind for 1 minute, and the affected creature makes a saving DC of 8 + proficiency + Wisdom when they get hit, and at the end of every turn while blinded (this move costs 2 ki points). At 11th level, this move does radiant damage equal to 2 of your martial arts die, and now blinds any 5 creatures (if the ball misses it still blinds creatures, and it blinds the closest creatures to it).

Spinning Tornado Kick

 As an action, starting at level 11, you can now stab your quarterspear into the ground and spin around it, your legs outstretched, and can make an unarmed strike (more specifically a kick) against every creature within 5 feet of you, as a bonus action (you spend 1 ki point to do this)

Ki Shuriken

Starting at level 11, as an action, you can now spend 2 ki points to form a shuriken made of pure ki. You can make a ranged attack with it against any, one creature within 60 feet of you dealing 2d6 piercing damage. You can additionally spend 4 ki points to throw 2 ki shurikens, which you can send each one at different creatures, or the same one.

KiJutsu Sensai

You've now mastered the arts of KiJutsu so well, that you can now fight better than you've been able to before. Starting at level 17, whenever someone makes a melee attack against you and misses, you can attack them with your quarterspear. You can use this ability once per turn, but not your own turn. You can also make an additional attack whenever you roll a natural 20 and get a critical hit (you can only do this once per turn). You can also use your quarterspear when doing acrobatics to give you an advantage on dexterity (acrobatics) checks. 

Ki Unleashed

Starting at level 17, you can spend 4 ki points to let your ki take over, completely consuming your body for 1 minute. Your body becomes pure ki giving you an advantage on all dexterity checks, and you're now immune to radiant damage. You also now do radiant damage along with the normal damage type, of your unarmed strikes. You emit bright light in a 50-foot radius, and dim light 20 feet beyond that. Your current speed gets increased, causing you to move twice as fast (eg. speed=100, now it equals 200).