The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
This weapon is decorated with dragon heads, claws, wings, scales, or Draconic letters. When it steeps in a dragon's hoard, it absorbs the energy of the dragon's breath weapon and deals damage of that type with its special properties.
Whenever you roll a 20 on your attack roll with this weapon, each creature of your choice within 5 feet of the target takes 5 damage of the type dealt by the dragon's breath weapon.
You gain a +1 bonus to attack and damage rolls made using the weapon. On a hit, the weapon deals an extra 1d6 damage of the type dealt by the dragon's breath 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.