The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
Choose a magical bonus of +1 to +3. This sword gains that bonus to its attack and damage rolls. For each point of bonus you choose for the sword, you take a corresponding penalty (−1 to −3) to your death saving throws. You can change this magical bonus each day at dawn.
This weapon is cursed, and becoming attuned to it extends the curse to you. Until the curse is broken with a remove curse spell or similar magic, you are unwilling to part with the weapon.
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.