Silent but deadly is no longer just a joke. The Ninja is a person skilled in the art of ninjutsu; the strategy and tactics of unconventional warfare, guerrilla warfare, and espionage. The functions of a ninja included reconnaissance, espionage, infiltration, deception, ambush, bodyguarding, and their fighting skills in martial arts.

Bonus Proficiencies

When you choose this archetype at 3rd level, you gain proficiency in the  Deception and  Stealth skills and poisoner's kit if you don’t already have them. Your proficiency bonus is doubled for any ability check you make that uses the [skill]stealth['skill].

Ninja Weapons

Starting at 3rd level, your training allows you to use Shurikens (dart), Kunai (dagger), and Blowguns as monk weapons. You gain proficiency with these weapons if you don’t already have them. When you use your flurry of blows class feature you can use these weapons for your attacks, regardless of whether they are melee, thrown, or a ranged attack.

One with the Shadows

By 6th level, you have honed your skill of slipping into the shadows to the level that you can do it in an instant. You can take the Hide action or apply poison to one of your weapons or piece of ammunition as a bonus action.

Silent but Deadly

When the Ninja is able to unleash deadly attacks on his foes when striking unexpectedly. Whenever the Ninja attacks with one of his monk weapons or unarmed strikes and has advantage on the attack roll, they score a critical hit on a roll of 19 or 20 and do additional damage equal to their martial arts die on a hit.

Death from the Shadows

At 11th level, you have learned the art of manipulating the visual senses of your foes and striking with ferocity after defeating your enemies. As a bonus action, you can turn invisible until the end of your next turn or until you make an attack or cast a spell. Additionally, once per turn when one of your attacks results in a creature becoming unconscious, you can make another attack as part of the same action using a monk weapon or unarmed strike.

Knockout Strike

At 17th level, you can use your ki to attempt to knockout your opponent when striking them. Once per turn, you can attempt to knock an opponent unconscious when you hit with an attack while using a monk weapon or unarmed strike and expend a ki point. The target must succeed on a Constitution saving throw or be knocked unconscious for one minute. They are woken early if they take damage, or someone uses an action to shake or slap the creature awake.