Terra Year 779, June, an "Apocalypse" level biohazard crisis emerged in the region surrounding Harrison Port in the Empire's Southern Ridge Province of the Setar Empire.
The swarm of giant parasitic worms, named "Heart-eater Worms," had already spread throughout the Empire's Southern Ridge and showed a trend of continuing to spread rapidly.
The infected would fall into a state of abnormal frenzy, then become vicious, turning into bloodthirsty beasts, seeking out any nourishment they could consume, and they were skilled at acquiring enough flesh and blood through killing.
Once they had acquired sufficient nourishment, the infected would enter a "transformation" state, enveloped by a huge flesh cocoon—those who metamorphosed would emerge from the cocoon, restored to their youthful prime, and possessing normal intelligence, but were completely controlled by the swarm, becoming the flesh shells of the Heart-eater Worms.
Or, in other words, a peculiar sort of semi-independent entity.