Its more along the lines that as he slowly becomes a vampire from having taken laura's blood, he gradually loses his humanity, and eventually, his humanity just fades altogether, leaving only well, his darkness. He sort of became a lord of shadow after all, in a different manner. Instead of ascending to heaven and leaving behind his dark side, he becomes a vampire and his humanity, his "good" just dies, leaving behind his anger, hate, and madness.
The Forgotten one seems to just be the final nail in the coffin, (pun not intended) rather, the final turning point of no return. When he just kills it despite it's pleas for mercy, and steals it's powers for himself, finally accepting what he has become, and leaving his former self behind with the symbolic act of destroying his Combat Cross, the weapon that ties him with his holy order and his fight for God. He basically shattered a Crucifix and tossed it away.
I think he mentions it in his narration, that he can feel his humanity ebbing away, and eventually his narrations just get kinda darker and he just outright starts calling himself the dragon and shit.