The ninja is a monk subclass. It focuses on achieving clarity in the field of combat through focus and exact movements. The purpose of a ninja is to infiltrate and kill people without being noticed.

To the world at large, the ninja do not exist. The tales of figures clad in dark clothing flying through the night are now the stuff of legend, and even then, the learned viewed such statements as little more than idiotic superstition, but the truth is more intriguing: the ninja still exist today, and they exist in you. Drawing upon the elements of nature, the ninja focus their skills into moments of supreme focus and clarity, aiding them both in and out of combat with quick maneuvers, deadly attacks, and preternatural celerity.

How did the ninja learn these skills? And how they draw upon these abilities? Does it come from achieving narrow focus through long periods of intense martial training, or do they access the reservoir of the stealth training you undertook. Whatever the method, the results attest to your achievement of skill and enlightened understanding.

The Earth Moves Beneath Me

Beginning at 3rd Level, your stamina and agility are un-paralleled. You are not impeded by difficult terrain, and your walking speed increases by 10 feet. This bonus increases when you reach higher levels, 15 feet at 6th level, 20 feet at 10th level, 25 feet at 16th level.

The Shadows Surround Me

Beginning at 6th level, A creature that you are hidden from is surprised when you cast a spell or make an attack. In addition, any hit you score against a creature that is surprised is a critical hit if you are in a dark or dim light area. You also gain expertise in stealth.

You may also attempt to hide in dim or dark light.

Death Flows Around Me

At 11th level, as an action you have the ability to make such an incredible number of strikes in such a beautiful manner at such incredible speed that you truly have mastered the dance of death. You may use your action to make one attack at advantage against an enemy within your range. If you land the attack it becomes a critical and you gain 3 ki points back. You may use this a number of times equal to your Proficiency bonus, you regain all uses at the end of a long rest.

The Void Shrouds Me

Beginning at 17th level, you can fade from sight. When a creature targets you with an attack, you may use your reaction to become invisible and gain +3 to your AC until the end of your next turn or until you attack or cast a spell. In addition, you can spend half of your base movement speed to remove the grappled or restrained condition from yourself.