For a normal ranker, even a blue core the pain from losing an eye, all your teeth and having your jawbone shattered would leave them in a considerable amount of pain. However, for a fiend they constantly live in pain that was part of the reason that they were feared. Those that lived in constant pain could deal with the pain from most injuries that they suffered during a fight.
Killing intent was like a poison for the mind as well as the body. Similar to how someone gets infected by another's affinity if they were stabbed, and it infected their body. For fiends it was as if they were burning alive at all times as killing intent ravaged their mind and body. That was the biggest reason why a majority of those that became fiends ended up losing their mind and becoming mindless killing machines until they were put down.
