Awesome Weapon for a Pyromancer.
Bonus Points for Fashon Souls.
For People like me who struggle with Vilhelm, because they want to get the Weapon early on:
I usually complete the game to the point untill you are under the Smouldering Lake (Izalith-Area), where you can grab Sacred Flame and Toxic Mist in the lava.
Both works for me with Vilhelm. Poison him first, kite and poke him outside of the building, and grab him with Sacred Flame when he trys to heal himself.
The Weapon itself deals really good damage in PvE.
Only some bosses are resistant against dark damage, like Twin Princes, where a Chaos infused Drakeblood Greatsword would be a better choice.