All sylphs have a soul that exists in an external object. When their human body dies, they return to their physical soul. However, if a sylph dies and their physical soul has been destroyed, their soul transforms into a fury: a sylph soul cannot survive without their soul home, but the destruction of a sylph soul is slow and agonizing. It is a bare nerve, skinless, exposed, until it finally loses all sense of humanity. It lashes out and destroys everything in its path.
Anatomy & Morphology
Furies have varied forms, however it's unclear what the different forms mean. Most often they look vaguely human shaped, but appear to be made out of black mud, with elongated limbs that they use to crawl around. They are incredibly fast.