Mordant Blade

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.

Source: Vault of Magic · OGL-1.0a · 5.1