The Shadowheart is a dark and immense epidemic released onto Kudao after the events of Yamamura. The effects include lung failure, crucial pain, and insanity. Vessels (a being infected by Shadowheart) can resist the hateful emotions and can stay somewhat sane to a degree, but those who embrace the hate can make the pain worse and worse but can get closer to a power found within the infection discovered by Kage. Those who embrace the Shadowheart are the Itami; a sadistic cult that continues to grow.
The disease can be caught by inhaling the air of Kindao, a hell not of our world. The first vessels sacrificed themselves to the infection to close the gateway for the sake of their homeworld. But despite their heroic sacrifice, all vessels were banished from Shilon after months of violent outbreaks. Some walk away from their past and start a new life while attempting to adapt to their torment. The rest have been converted into the Itami cult, and have become the most terrifying beings among Kudao.
The infection beginning to grow.