The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
You have a +3 bonus to attack and damage rolls made with this magic weapon. When this weapon reduces a creature to 0 hit points, the creature doesn't die. Instead, infernal runes appear as if carved into their flesh and they're healed to 1 hit point. They now have the paralyzed condition until the condition is removed by a Lesser Restoration spell or similar magic. When the condition is removed, the runes disappear. At the start of each of their turns while they're paralyzed, the creature suffers immense pain and gains a level of exhaustion.
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.