A Dormant Darkness

"AVADA KEDAVRA!" "Mr. Potter!" "He can bewitch you and drain you of every last drop of humanity you have left!" "Harry [...] I don't want to lose you". A terrible accident will disrupt the life of Harry Potter and irrevocably those around him. An unexplained rage and some personality changes will transform him. Will anyone be able to contain him or will Harry Potter lose himself?