Sword of the Sea King

The title is just that, a title. It doesn't actually belong to Neptune or anything. although, He would probably like to have it due to it's ability.

weapon name: Leviathan
description: A long Katana with a rather wide blade, and a sea green tint to the blade due to it being made of seastone. Has a hilt like a Sea serpent head swallowing the blade, and it has a long handle in the shape of a Sea serpent's tail.
weapon special ability's: Cannot dull(because Katana's don't get dull-or at least i heard) Can cut a logia devil fruit user like they were human, cancels out any devil fruit power due to being seastone. Can also theoretically calm a sea king, but it hasn't been tested.
history: Forged at the beginning of the world government, this sword has been passed down through a Blacksmith family for ages, until the latest descendant decided to actually use it against a corrupt Devil fruit using marine who was attacking his town.

why would it be able to calm a sea king? and katanas can dull? but well it doesnt seem too much of an issue