A speedster has tapped into a mysterious power that has granted them incredible speed, allowing them to move faster than the eye can follow.

Superior Reflexes

You can perceive and react to danger faster than the blink of an eye. You add your wisdom modifier to initiative and attacks of oppurtunity against you have disadvantage.

Inhuman Speed

You are capable of moving at incredible speeds when you need to. The dash action gives you extra movement equal to twice your speed, after applying any modifiers, rather than the usual amount.
This also applies to the dash action granted by your Step of the Wind feature or any other feature.

Additionaly, your base movement speed increases by 10ft and your unarmored movement increases by an extra 10ft.

The effect of becoming a Speedster also results in your body needing more energy to keep up with your speed use. When you eat, you need to consume twice as much as normal to feel sustained. If you go a day without eating as much as you should, you are under the effects of the Slow spell until you complete a short rest to eat the amount your body needs.

Mach Punch

Due to your immense speeds, your punches are devastating.

Your unarmed strikes deal an extra 1d4 bludgeoning damage. This extra damage increases to 2d4 at level 11 and 3d4 at level 17.

Molecular Vibration

You are able to vibrate your molecules at such speeds that you can pass through solid objects. When you use your Step of the Wind feature, you can also move through creatures and objects as if they were difficult terrain until the end of your turn. If you end your turn inside a creature or an object, you take 1d10 force damage and are shunted to the last unoccupied space you occupied.

Fast as Light

At 17th level, you begin to move so fast that everything around you appears slow in comparison. You can spend 4 ki points as an action to cast haste on yourself without material components. In addition, whenever you take damage from a creature within 5 feet of you, you can make an unarmed strike against that creature or you can spend 2 ki points to halve the attack's damage against you.