Warning: dragon-sized Game of Thrones spoilers ahead!
Since the moment they were hatched from three fossilized eggs in season one, Daenerys's dragons have been a constant source of wonder, fear, and awe in Game of Thrones. Without her three children, it's safe to say that the self-proclaimed Queen of the Seven Kingdoms wouldn't have anything she has today — her army of Unsullied and Dorthraki, her retinue of faithful followers, and her claim to the Iron Throne. As Daenerys's power grew, so did Drogon, Rhaegal, and Viserion, and we watched as they did everything from protect their mother to raze her enemies. But it hasn't all been white-hot flames and breaking chains. We've had to watch Daenerys mourn the loss of Viserion to the Night King in season seven, and, after season eight's fourth episode, Rhaegal joins his formerly possessed brother in death, all thanks to Cersei and Euron's battle tactics.
In the aftermath of the Battle of Winterfell, Daenerys is finally able to lay Viserion and Jorah to rest, as well as bid adieu to any threat of the Night King. She decides to look forward to her future on the Iron Throne by making her way South to King's Landing to finally contend with Cersei, Euron, and the Golden Company.
As Daenerys flies on the back of Drogon (who's looking a little torn at the wings after his close call in the last episode) back to Dragonstone, Rhaegal flies beside them. It's a sweet moment for the four seconds of happiness Daenerys is able to grab before a spear flies out of freakin' nowhere and stabs Rhaegal in the chest. Turns out Euron's fleet is hiding behind some rock formations, ready for the chance to take aim at the Dragon Queen and her remaining two children. He launches another few spears and Rhaegal goes down, another child of Daenerys's lost to her war.
Most of the rest of the retinue is able to get to safety, but not without another casualty to follow. It turns out Cersei sent Euron as a message to Daenerys and it's one that the younger Queen definitely receives. Tyrion manages to curb Daenerys's rage enough to get her to agree to give Cersei one last chance to surrender and return Missandei before she lays siege to King's Landing, but as we soon see, it doesn't work out in her favor.
Now Daenerys is down to one dragon and has lost all her closest allies, and the war for the Iron Throne has begun in earnest. And with Rhaegal dead, Daenerys's advantages shrink down to one very vulnerable remaining dragon. At this point, we have no doubt that Daenerys is going to go full Targaryen on King's Landing — what does she have to lose now? She's basically lost everything she's loved and now that she's so close to getting the Throne — she has nothing to hold her back. From the look on her face at the end of episode four, things are definitely about to get fiery.