"Fire cannot kill a dragon."
Once believed to have gone extinct, Daenerys Targaryen’s three dragons — Drogon, Rhaegal, and Viserion — were fierce, powerful creatures. Armed with scales that shielded them against the most lethal attacks, the dragons could decimate entire cities with a simple “Dracarys” command from Daenerys.
It shouldn’t have been possible for Game of Thrones to kill off Daenerys’ dragons so easily. Euron Greyjoy quickly dispatched Rhaegal in Game of Thrones Season 8, shooting a scorpion dart through his underbelly. In Season 7, the Night King killed Viserion by launching an ice spear through his neck. It was disappointing to see these immensely powerful creatures one-shotted after hearing so much about their legendary power. But the deaths of Rhaegal and Viserion don't align with the lore within George R.R. Martin’s A Song of Ice and Fire. The Winds of Winter, the sixth book in the series, could fix the biggest dragon problem from Game of Thrones.
How to kill a dragon
Thousands of years before Viserion, Rhaegal, and Drogon were born, dragons populated most of the world, in Westeros and beyond. It’s unclear when and where dragons originated — be it from the Shadow Lands or Asshai — but at one point they were brought over to Valyria.
The Valyrians trained the dragons, learned how to ride them and turned them into weapons of destruction. They used the fearsome dragons to fight wars and conquer empires. Following the Doom of Valyria, a mysterious event that collapsed the Valyrian civilization and killed most of the dragons, the Targaryens became the sole owners of dragons.
Despite their dwindling numbers, dragons were nearly invulnerable. They were immune to fire and the older they grew, the thicker and tougher their outermost scales became. They lived for hundreds of years and their flames could even burn stone.
However, when the Targaryens, now fierce dragon riders, attempted to conquer all of Westeros, the Dornish resisted. Paralleling what Euron did in Season 8, the Dornish warriors shot a scorpion bolt at the dragon Meraxes, killing the dragon instantly. Crucially, the difference was that the bolt went straight through Meraxes’ eye and not through the chest.
The next wave of dragons died during the Dance of the Dragons, a Targaryen civil war, over a hundred years later. This event began the downfall of House Targaryen — no dragons, no power. Dragons began dying left and right, mostly in combat with each other. However, another foe shot a crossbow into a dragon’s eye to kill it. You see the pattern here, right? In short, Euron would likely have not been able to kill Rhaegal unless he pierced the dragon’s eye with his scorpion dart.
Shooting a dragon through the eye isn't the only option. Some were killed by mobs, who went after the dragons chained down in a pit in King’s Landing during the Dance of the Dragons. It’s also been established that dragons can die by drowning. They need light, nourishment, and freedom. Locking them up made them small and sick and it wasn’t long before the last of them died out.
On the other hand, Archmaester Marwyn was under the impression that the maesters wanted the dragons gone because they didn’t like magic and how connected the dragons were to it. Fan speculation also contends that the Citadel may have poisoned the remaining dragons over time before they went extinct.
Dany's children deserve better
Daenerys’ three dragons are still alive in the books, in Mereen. There’s no telling what Martin’s plans for the dragons are once she returns, but don't expect Rhaegal and Viserion to be killed off so haphazardly.
The Night King doesn’t even exist in the books yet, which is good news for Viserion. That said, Euron seems to have it out for Daenerys. He wants power first and foremost and sends his Iron Fleet to find Daenerys and bring her and the dragons back to Westeros. Euron even sends a dragon horn to Mereen as a “gift,” though it’s full of poison. Knowing how badly he wants the Iron Throne, Euron could still kill Rhaegal.
If Rhaegal is to ultimately die in The Winds of Winter, it should definitely be more of a climactic event than on the show, especially considering that Euron was a relatively new character who just happened to take a random shot at a dragon. It's possible Euron will attempt to figure out how to kill dragons so that he can take the perfect shot, likely through the eye.
There's a slim chance that none of the dragons will die, but Daenerys' claim to the Iron Throne makes them an obvious target for her enemies. The very least The Winds of Winter could do is give Rhaegal and Viserion a more memorable sendoff.