How I'd rule:
1) Bonus action: Fiery blade in your free hand, shaped like a scimitar, lasts 10 mins, disappears if you let it go but reappears with a future bonus action. It sheds light.
2) You can (but not must) use your action to make a melee spell attack with it, and a hit does 3d6. A melee spell attack is not "taking the attack action" so no additional attacks can be made if you choose this option.
3) You can (but not must) instead use your action to take the attack action. You can make a melee attack with the fiery blade shaped like a scimitar. Since it is not an actual scimitar, just an object evoked to be shaped like one, it is an improvised weapon. Unless you have the Tavern Brawler feat or something which gives similar benefits, your attack will not include your proficiency bonus because, while it might look like a scimitar, it's really malleable flaming energy with a pommel. On a hit "the DM assigns a damage type appropriate to the object" and in this case the damage is 3d6. You can use extra attack to attack again.
So if you want to use extra attack, you have to use the improvised weapon rules, and the object is so unusual you won't be adding your proficiency bonus to the attack. But you will be getting the 3d6 damage per successful hit, and you will get your extra attack.