One of Daenerys Targaryen's dragons suffered a ghastly fate in Episode 6 of Game of Thrones Season 7. Their size and power may have made the dragons seem impossible to defeat, but it turns out they can be taken down after all. The White Walkers must have powers beyond comprehension, because just one of their icy spears was enough to end Viserion's life. He was resurrected as a wight, and it's clear he's going to be a very different dragon going forward. But what changes could this transformation bring about? Do ice dragons breathe ice or fire on Game of Thrones?
Viserion hasn't exercised his new abilities on the show yet, but Westerosi lore answers this question nonetheless. According to George R. R. Martin's A Song of Ice and Fire series, as well as its companion book The World of Ice & Fire: The Untold History of Westeros, ice dragons do indeed breathe freezing cold air instead of fire. In an excerpt from the companion book, it says, "Whereas common dragons (if any dragon can truly be said to be common) breathe flame, ice dragons supposedly breathe cold, a chill so terrible that it cane freeze a man solid in half a heartbeat."
And in the books, specifically A Dance With Dragons, Jon Snow remarked on how the wind was "cold as the breath of the ice dragon in the tales Old Nan had told when Jon was a boy." It looks like ice dragons really know how to stay on theme.
But Viserion isn't quite like those mythological ice dragons. Despite the fantastical qualities of those creatures, they were still living things; they just preferred colder climates. Viserion was a regular dragon who was killed and reanimated. There's no telling how much he'll have in common with the ice dragons of lore, though he must be able to possess some impressive new skills or the White Walkers wouldn't have bothered to haul him out of the lake he fell into after being killed.
And it would make sense that, like more traditional ice dragons, Viserion may be able to produce frost instead of flames. The White Walkers are all about the ice, so a wight creature being able to breathe extreme cold would be more logical than Viserion still being able to breathe fire. That wouldn't be of much help to the Walkers. His being able to produce ice, however, could be incredibly beneficial; if Walkers can only function in the cold, then they could use their new ice dragon to freeze the path ahead of them, bringing down the temperature throughout Westeros as they advance further south.
But that's just speculation. The only thing that's certain is that Viserion has changed — while he was once a helpful ally, now he's going to be a powerful enemy.