Quelaag's Furysword

Can be achieved by having the soul of Quelaag and a Shotel or another Curved Swords +10. Go to the blacksmith in Anor Londo with these items and he will smith the fury sword for you.

In-Game Description
A curved sword born from the soul of Quelaag, daughter of the Witch of Izalith, who was transformed into a chaos demon. Like Quelaag's body, the sword features shells, spikes, and a coating of chaos fire. Power affected by wielder's humanity.

Overview
Quelaag's Furysword is a fast, light weapon, capable of quickly racking up damage in exchange for low staggering power. Unusually, Quelaag's Furysword deals mostly fire damage, although it can still deal a decent amount of physical damage when upgraded.