I should note that the only way to kill a demon is with the poison of a very rare flower that grows at the tops of a few volcanoes throughout Terra. Over half of which are in Xi'an, with most of the rest in Ocrad.
It slows down their ability to heal, making them mortal for a few hours. It's very expensive, but even just a few drops makes a wound unhealable for a few hours.
Demons use it to kill their own traitors, and to make humans incurable by magic.
In other words it stops all magic healing until the immune system neutralizes it