While The Hound has spent years as an outsider on Game of Thrones, and spent a couple of seasons in hiding, it looks like he'll be getting back into battle soon. His mind was changed by a vision he had, but what did The Hound see in the fire in the Game Of Thrones Season 7 premiere that caused him to express genuine terror for the first time?
The Hound believed that men live and that men die, and nothing else really factored into his understanding of the world. However, his current compatriots deal in things far more complicated than life and death. Beric Dondarrion has died and lived many times, and Thoros of Myr promises to have a direct line to the Lord of Light. The Hound has grown more accepting of faith after his time in a commune, but what Thoros of Myr shows him in the fire is far beyond anything The Hound could've expected to see.
His vision begins at the Wall, specifically where "the Wall meets the sea." He then sees a castle there. The fire cracks. He sees a mountain that looks like an arrowhead. He sees the dead. He sees thousands of the dead, an entire army's worth and then some. Beric asks "Do you believe me now, Clegane? Do you believe we're here for a reason?"
The Hound's vision in the fire is similar to the vision that Bran has at the very beginning of the episode. Bran sees an army of wights, including giants, led by The Night's King. They're marching south to The Wall and seem to be intent on taking everything that lies beyond it.
The look of horror on The Hound's face says it all. He can't just make a sarcastic comment and swing his sword like he usually does. This is a problem far larger than him or his ego. But what can he do about it? Where will he go next? Where is this mysterious mountain and what castle did he see where The Wall meets the sea?
The castle he describes is a perfect match for Eastwatch-by-the-Sea, a station on The Wall. Eastwatch was mentioned earlier in the episode, when Jon Snow commanded the Wildlings to go there and fend off any White Walkers that may approach. Game of Thrones famously has a major battle almost every season, from the Battle of the Blackwater to the Battle of the Bastards. It looks like Season 7 is gearing up for the Battle of Eastwatch, and The Hound may be a major player in this fight against the dead.
And what about the mountain that looks like an arrowhead? Every Game of Thrones fan that loves theories jumped at the mention of a Mountain, as it's the adopted name of his brother-turned-monstrosity and the other half of the famed Cleganebowl. Whether the mountain that The Hound saw is his brother or an actual mountain, this vision promises that things are about to get very violent very fast for Sandor Clegane.