The Double-Bladed Scimitar can be found in Eberron: Rising from the Last War, page 21.
When you attack an object with this magic sword and hit, maximize your weapon damage dice against the target.
When you attack a creature with this weapon and roll a 20 on the attack roll, that target takes an extra 14 slashing damage. Then roll another d20. If you roll a 20, you lop off one of the target's limbs, with the effect of such loss determined by the DM. If the creature has no limb to sever, you lop off a portion of its body instead.
Note: According to the SRD, it is an extra 4d6 slashing damage, although this is incorrect.
In addition, you can speak the sword's command to cause the blade to shed bright light in a 10-foot radius and dim light for an additional 10 feet. Speaking the command word again or sheathing the sword puts out the light.
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.