the hound "died" (figuratively) when he nearly died. in the show, he doesnt have his helm the entire time, because they want to show his face. but in the books, the symbolism is that he is saved by the septon (rey in the show), and he left behind his dog head helm
so the hound died. he's a broken man. but a healing man
clegane bowl was an emotional struggle in his heart (theres a scene from season 4 episode 3 where he tells arya "the worst part wasnt the pain, it was that my brother did it"
so the hole was that he never had family. but now, he does. sansa, arya, beric, thormund, and others
Arya will kill the Mountain in season 8