The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
You have a +1 bonus to attack and damage rolls made with this weapon.
When you roll a 20 with this magic weapon, the target takes an extra 7 Slashing damage.
Note: According to the SRD, it is an extra 2d6 Slashing damage, although this is incorrect.
If you attack with a double-bladed scimitar as part of the Attack action on your turn, you can use a bonus action immediately after to make a melee attack with it. This attack deals 1d4 slashing damage on a hit, instead of 2d4.
This weapon requires two hands to use. This property is relevant only when you attack with the weapon, not when you simply hold it.