Ascendant Nightscale's Tailblade

This weapon is decorated with dragon claws, scales, and Draconic letters. 

Slumbering (Uncommon). 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 acid damage.

Stirring (Rare). The Stirring weapon has the Slumbering property. In addition, you gain a +1 bonus to attack and damage rolls made using the weapon. On a hit, the weapon deals an extra 1d6 acid damage.

Wakened (Very Rare). The Wakened weapon has the Slumbering property, and it improves on the Stirring property. The bonus to attack and damage rolls increases to +2, and the extra damage dealt by the weapon increases to 2d6 acid damage.

Nightscale's Wrath - As an action, you can unleash a 30-foot cone of destructive energy from the weapon. Each creature in that area must make a DC 16 Dexterity saving throw, taking 8d6 acid damage on a failed save, or half as much damage on a successful one. Once this action is used, it can’t be used again until the next long rest.

Ascendant (Legendary). The Ascendant weapon has the Slumbering property, and it improves on the Stirring and Wakened properties. The bonus to attack and damage rolls increases to +3, and the extra damage dealt by the weapon increases to 3d6 acid damage. The cone of destructive energy the weapon creates increases to a 60-foot cone, the save DC increases to 18, and the damage increases to 12d6.

Proficiency with a Longsword allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Acid, Bonus: Melee Attacks, Damage: Slashing, Weapon Property: Finesse, ritual of infusion with draconic essence, Damage, Combat, Versatile, Sap