Sandor “The Hound” Clegane, whom we last saw back in season four, was badly injured in a brutal fight with Brienne, and Arya apparently left him to die alone.
But Sandor aren’t easy to kill, and reappears in this episode: the first, that I remember, that starts without the loved-hated the opening theme.
V ox.com has published a good article about The Hound’s return, i suggest the reading:
The younger son of a minor Westerlands family, Sandor was first introduced to us as a menacing warrior who served the Lannisters generally, and Cersei Lannister’s son Prince Joffrey specifically. His face was hideously burned during childhood by his abusive older brother Gregor (“the Mountain”), leading to a lifelong fear of fire.