Instructions
In World 1, get to the very top of the gate that the Fat Official has closed off in the beginning. When you cross the bridge, kill the Red Eye Knight patrolling the area. Turn right and vault over the rampart towards the castle; you will land on a ledge with a body with the sword nearby.
Also in World 1, you can obtain the sword by defeating the Penetrating Sword Black Phantom. You can lure him out individually so you don't have to fight three of them at once.
Enjoy the weapon's long reach and increased thrusting attack damage.