The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
You gain a +3 bonus to attack and damage rolls made with this magic weapon.
The first time you attack with the sword on each of your turns, you can transfer some or all of the sword's bonus to your Armor Class, instead of using the bonus on any attacks that turn. For example, you could reduce the bonus to your attack and damage rolls to +1 and gain a +2 bonus to AC. The adjusted bonuses remain in effect until the start of your next turn, although you must hold the sword to gain a bonus to AC from it.
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.