Morbid: The Seven Acolytes

While exploring in Grimwald's Grove, you will come across a giant version of the broccoli monsters, but this one is already dead. Right next to it you will see a body you can loot. This will uncover the Blader weapon for you and unlock the achievement. The video below shows its location using one of the shrines as a reference point. Since there is no map to simply circle the spot, that's the best way to find it.