He once was a warrior of purity wanting to fight off the End. But when he entered the End, he was over powered by the darkness that was in it. It ran through his body like a posion, coruppting him and changing in to this vile man. When he came back to the surface world he was found on the ground by some villagers. They happen to know the cure for this poison of this darkness, liquid emerald, but at this point it was impossible to try and heal to his original self. So they have a constant supply of the liquid emerald going through his body.