The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
The black blade of this sword is crafted from a mysterious arcane alloy. You gain a +1 bonus to attack and damage rolls made with this magic weapon. While the sword is on your person, you are immune to effects that turn undead.
While holding the sword, you can use an action to give yourself 1d4 + 4 temporary hit points. This property can't be used again until the next dusk.
When you hit a creature with an attack using this weapon, you can fill the target with unsettling dread: the target has disadvantage on the next saving throw it makes before the end of your next turn. The creature ignores this effect if it's immune to the frightened condition. Once you use this property, you can't do so again until the next dusk.
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.