Warning: Contains spoilers for House of the Dragon season 2 and the book Fire & Blood.One House of the Dragon season 2 death has already been revealed, and the spoiler confirms how the show will pay off a big change season 1 made to Fire & Blood. House of the Dragon season 1 stuck to the broad strokes of the story outlined in George R.R. Martin's book, but by necessity (since it's written as an in-universe historical account) it also needed a lot of invention. Some of these were minor, but others were definitely major - Laenor Velaryon still being alive; Aemond Targaryen killing Lucerys Velaryon by accident - and the consequences of those decisions will play out further in House of the Dragon season 2's story.
The latter, in particular, is the launchpad for everything that's to come, as it's what truly causes the Dance of the Dragons to begin. The Targaryen civil war will play out across however many seasons House of the Dragon has left (likely 3 more, including season 2), and with that comes big battles and bigger deaths. Humans and dragons alike will perish, and set photos and videos have already spoiled one major casualty, with Meleys' House of the Dragon death confirmed.
Meleys' death comes at a key point in the Dance of the Dragons, with the blacks in the ascendancy having taken the crucial castle of Harrenhal, as well as many houses having joined Rhaenyra's cause. King Aegon II Targaryen makes drastic changes, replacing Otto Hightower as Hand of the King with Ser Criston Cole, and one of the first results of that is the Battle at Rook's Rest. Rook's Rest was the home of House Staunton, situated in the crownlands north of King's Landing. Criston decided to march almost 3,000 men north, forcing Houses Rosby and Stokeworth into submission before targeting Staunton.
Staunton shut himself and his soldiers inside Rook's Rest, and sent word to Rhaenyra to ask for help, and just over a week later it arrived in the form of Rhaenys Targaryen and her dragon, Meleys. Their arrival initially looked to turn the tide, even though Criston was ready for it, with archers firing arrows upon them. It was then, however, that Criston sprung his trap, in the form of Aegon and Aemond and their dragons, Sunfyre and Vhagar.
The two dragons met Meleys in the air, tearing and breathing fire at one another. Although the greens suffered casualties, and Sunfyre and Aegon were gravely injured, Rhaenys and Meleys were both killed in the fight, unable to overcome the numerical disadvantage. The dragons fought and went tumbling to the ground, killing many below, but Meleys would be the one left dead, and Rhaenys with her. Meleys' head was then taking to King's Landing, as seen in the House of the Dragon season 2 set photos.
House of the Dragon season 1, episode 9 made a dramatic change to the book, with the addition of Rhaenys and Meleys at the coronation of King Aegon II Targaryen. Although Rhaenys decided not to kill any of the greens, the sheer amount of destruction she caused likely meant at least a few hundred smallfolk died because of her actions, something that was always likely to have consequences. At the time, that seemed to be laying groundwork for the storming of the Dragonpit, an event later in the Dance when the people of King's Landing break in and kill the dragons inside the pit. That still holds true, but Meleys' death pays it off earlier.
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The House of the Dragon season 2 set photos and videos show that Meleys' head will be paraded through King's Landing, and it's clearly a cause for celebration. It highlights how Rhaenys' maneuver backfired, with the people of King's Landing cheering the dragon's death, its head a welcome sight to those who lost people because of it or at the very least were terrified in the Dragonpit. With the greens needing to win support, and make sure people don't trust Rhaenyra, then this is a key part of their campaign, which also fits with events that will see the Black Queen proclaimed "Rhaenyra the Cruel" in King's Landing.