Mordant Longsword 2014
Weapon, rare, (requires attunement)
You can use a bonus action to speak this magic weapon's command word, causing the blade to weep a caustic, green acid. While weeping acid, the weapon deals an extra 2d6 acid damage to any target it hits. The weapon continues to weep acid until you use a bonus action to speak the command word again or you sheathe or drop the weapon.
Properties
Versatile (1d10)
This weapon can be used with one or two hands. A damage value in parentheses appears with the property—the damage when the weapon is used with two hands to make a melee attack.
Source: Vault of Magic (Kobold Press) · License: OPEN GAME LICENSE Version 1.0a details