The ninja is classically depicted as an assassin of great skill and strong will. Able to move through a targets home undetected via unusual routes and quick execution of any witness; Able to find it's way out of unfavorable situations using special tools specifically designed for subversion and disabling; Able to use it's unusual weapon set to great effect in catching it's targets off-guard. Using poisons, illusions, and stealth, the ninja makes it's way in the world through trickery, subtlety, and the use of cold hard steel.

 

Weapon Flurry

 

Starting at 3rd level, whenever you use Flurry of Blows or your Martial Arts feature for a Bonus Action Attack on your turn, you can use ninja weapons instead of only your unarmed strike. Ninja Weapons include the Dart, Dagger, and Sickle. This does not interfere with your ability to use other weapons for your Attack Action.

You also gain proficiency in longswords (Katana) and they count as monk weapons for you. Additionally, you also gain Blindsense of 15ft

Ninjitsu

Starting at 3rd level, you can spend Ki points to use these effects as a Bonus Action.

 

Vanish: 1 Ki Point- A cloud of smoke appears until the end of your turn. It is centered on you, has a 10 ft radius, and does not move with you. Everyone in the radius, aside from you, is considered Blinded.

Erratic Movement: 2 Ki Points- Cast Water Walk or Spider Climb on yourself. Must maintain concentration normally. This version lasts 10 minutes.

Multiform: 5 Ki Points- Cast Mirror Image. Can attack an additional time when you take the attack action while at least one duplicate is still active.

you can additionally use your ki to duplicate the effects of certain spells. As an action, you can spend 2 ki points to cast silence, Darkness, or pass without trace without providing material components. 

Shadow Walk

Starting at 6th level, you can tap into the shadows that you hide in to become one with them. Spending 3 Ki and an Action, you and everything you carry become surrounded by a thick Black Fog and become a shadow.

Once in Shadow walk your movement speed is halved, your Blindsense is increased to 30ft.

You gain darkvision out of a range of 60ft, if you already have darkvision increase it by 30ft.

You also gain the ability to push through walls. Walls count as difficult terrain, and can stay inside a wall as if you are holding your breath. Anything that isn't worn or carried stays the same and can't be moved through walls

You leave no sound or evidence of being there, you gain a +5 on Stealth. When in dim light or darkness, you gain advantage on stealth.

This ability lasts an hour, and can only be used in dim or darker light. and if in any wall when the duration ends, you get knocked prone next to the closest nearby space.

Rushing Escape

Starting at 11th level, when a creature moves adjacent to you, you may use your reaction to make 1 attack with advantage against that creature, if your Lethal Strike ability is not active when this attack is made, you may spend 1 Ki Point to activate it. After making this attack, you may move up to half your speed without provoking reactions from the creature you attacked

Lethal Strike

Starting at 17th level, as a bonus action, you can use 6 ki points to apply poison to all of your weapon attacks for the next minute. This does 4d6 poison damage. An enemy hit by this for the first time this turn must make a constitution saving throw or gain the poisoned condition until the end of their next turn.