The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
You gain a +1 bonus to attack and damage rolls made with this magic weapon.
When you hit an undead creature with an attack using this weapon, the attack deals an extra 1d8 damage of the weapon's type, and the creature has disadvantage on saving throws against effects that turn undead until the start of your next turn.
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.