Weapon (scimitar), very rare (requires attunement)
You gain a +2 bonus to attack rolls and damage rolls made with this magic weapon. In addition, you can make one attack with it as a Bonus Action on each of your turns.
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: Bonus: Magic, Extra Attack as Bonus Action, Damage, Combat, Finesse, Light, Nick
So three attacks a turn? Base, Nick, Speed
The Nick attack needs to be with a different weapon, but yes.
There is nothing in the rules saying the Scimitar of Speed needs to be in your main hand to get the bonus action attack. As long as you have a Light weapon in your main hand you can do Action: [Main hand attack + Offhand Scimitar of Speed Nick] Bonus Action: [Offhand Scimitar of Speed Ability], you'd just not be able to add your Str/Dex to the damage unless you have Fighter Two-Weapon Fighting.
So, let's say you're wielding this and a shortsword. You Booming Blade, then attack with this on your Bonus Action. Would you then get to use the Nick property from your Bonus Action attack to attack again with your shortsword?
Overlooked feature of this weapon is that, unlike most sources of Bonus Action attacks, this one's property doesn't require you to spend your main action attacking. This is great on any gish who might want to spend their main action casting, and has additional utility on any martial, as you can Dodge/Dash/Disengage as main action and still attack
In fact, combined with the Slasher feat, you can BA attack, reduce enemy speed, and disengage safely - great way to put some distance away from an enemy with similar movement speed