Does the Lord of Light show Sandor these visions to aid their quest or spell an inevitable armageddon for all of mankind? What does he really see? The secrets may lie in Beric's position in this mess.
"Poor Beric Dondarrion, who was set up as the foreshadowing of all this," author George R.R. Martin told Time in a lengthy and revealing Season 7 interview. "Every time he’s a little less Beric. His memories are fading, he’s got all these scars, he’s becoming more and more physically hideous, because he’s not a living human being anymore. His heart isn’t beating, his blood isn’t flowing in his veins, he’s a wight, but a wight animated by fire instead of by ice, now we’re getting back to the whole fire and ice thing.
Martin positions Beric, more man than we'd ever consider The Night King, to be the fiery inverse of your average White Walker (which explains his abilities to conjure a fire sword, as seen in the trailers for Season 7). Perhaps Sandor, embracing the fire half of this dual god, is the only force that can wallop the White Walkers in hand to hand combat? Not only will the Brotherhood without Banners travel to Eastwatch-by-the-Sea to face their fate, they'll now do so in the context of a religious war, where the forces summoned by light and dark, fire and ice, square off. Melisandre's quote from Season 4 only makes the god's involvement in the showdown all the more ominous: "There's only one hell," she tells Shireen Baratheon, not yet burnt at the stake in honor of this Lord. "The one we live in now."