While holding this magic weapon, you can take a Bonus Action and use a command word to cause flames to engulf the damage-dealing part of the weapon. These flames shed Bright Light in a 40-foot radius and Dim Light for an additional 40 feet. While the weapon is ablaze, it deals an extra 2d6 Fire damage on a hit. The flames last until you take a Bonus Action to issue the command again or until you drop, stow, or sheathe the weapon.
Proficiency with a Scimitar allows you to add your proficiency bonus to the attack roll for any attack you make with it.
This weapon has the following mastery property. To use this property, you must have a feature that lets you use it.
Nick. When you make the extra attack of the Light property, you can make it as part of the Attack action instead of as a Bonus Action. You can make this extra attack only once per turn.
Notes: Damage: Fire, Damage, Combat, Finesse, Light, Nick
Cool!
Very Cool
So once its active you do 1d6 (regular) + Proficiency modifier + 2d6?
Not quite. The proficiency modifier only adds to your chance to hit, not damage (expect in some special cases like Hexblade's Curse)
Instead, the damage would be 1d6 Slashing + Dexterity or Strength modifier + 2d6 Fire (when active)