Nearest I can tell, when Godwyn was killed his soul was annihilated while his body was left alive but tainted with the ontological force of real, actual Death that had been separated out of the fundamental rules of the world to create reincarnation and resurrection and allow for true immortality for the privileged nepo babies of the Golden Order; funeral rites were performed for him and his body was interred in the roots of the Erdtree, which normally would have led to him being reincarnated or resurrected but instead he just lay there, lifeless but alive (since his soul was gone completely, annihilated or otherwise entirely removed from the world and its rules), and tainted the roots with a new sort of paracausal force that spread out through the root network and began interrupting the cycle of reincarnation by making soulless bodies rise up and live again despite their state (and I think this entails destroying their souls as well, as Godwyn’s was, because of how this is treated as more than just defiling empty bodies but as something existentially vile).

Deathroot is the concentrated focal point of a local infection, and Gurranq was bound using the fundamental ontological rules of the world and his own nature as the fated shadow of Marika to hunt down and destroy all of the outbreaks by literally consuming it, a task which is impossible because the infection is inaccessibly deep and constantly propagating itself.