Blinding Speed increases your speed by 10ft., and gives you a number of "speed die" equal to your proficiency bonus + dexterity modifier. These die are d8, and can be used in the following ways:
- Speed Amplification: As a bonus action, you can spend any number of speed die, gaining movement speed equal to 5x the result of the roll until the beginning of your next turn.
- Quickened Assault: When you take the attack action to make a weapon attack, you may spend speed dice. Make one additional weapon attack for each speed die spent. You deal 1d8 damage with these additional attacks, and do not add your ability modifier.
- Blink of an Eye: As a bonus action, you may choose a location you can see within your movement speed range, and spend one speed die to teleport to the chosen location. If you make a melee weapon attack against a creature within 5 feet of you after teleporting, roll 1d8 and add the result to your attack roll. Subtract the distance teleported from your remaining movement in that turn.
- Flash of Defense: When you take damage from a creature as a result of an attack that targeted you, or dexterity saving throw you were forced to make, you may spend any number of speed dice, reducing the damage dealt by the number rolled.
In addition, enemies have disadvantage on attacks of opportunity made against you.
(This feat is not intended to be available under normal circumstances, such as via an ASI or racial feature. If used in that way, it is incredibly unbalanced and will probably ruin the game.)