Crona is Medusa's child, who is abused and experimented on in an attempt to make Crona into a demon god. She instills upon Crona obedience and madness as a child by forcing Crona to kill small animals; thrown into a dark room for days as a punishment for disobeying. She also replaces Crona's blood with "black blood", a substance that hardens to protect Crona from physical damage. Ragnarok , a demon blade, is also melted into the blood, which gives him a symbiotic relationship with Crona. He constantly picks on and belittles Crona, which reinforces Crona's shy and naive personality. Crona finds it impossible to interact with other people, often hiding in dark corners to ease anxiety.
Source from Wikipeida
black blood, physical damage, symbiotic relationship