House of the Dragon: Who Will Slay Rhaenyra Targaryen?
When it comes to House of the Dragon, as with Game of Thrones before, saying that a character gets killed at one point is hardly a spoiler – it is more common than surviving to the end.
Rhaenyra Targaryen, the Black Queen, gets killed at one point of the Dance of Dragons, the civil war which started in the final episode of House of the Dragon Season 1.
It is more of a spoiler to reveal the circumstances of her death, which were pretty ignominious, and the identity of her slayer.
Specifically, at one point during the war her Black party managed to take King's Landing, and Rhaenyra herself moved her residence there. But, going by Fire and Blood, the latest and longest version of George Martin 's history of Targaryen dynasty, her rule was remembered as brutal and inept, beset with the vicious circle of Queen's paranoia towards her underlings and betrayal on the part of said underlings.
Rhaenyra was deeply unpopular among the city's denizens, and when her half-sister and prisoner, Helaena Targaryen, died under suspicious circumstances (though the chronicler, writing the story in-setting does not believe Helaena's death to be murder), discontent boiled over into one of the most savage revolts in the history of Westeros.
Rhaenyra's dragon, Syrax, was slain in the fighting, and Rhaenyra herself had to flee the capital with an excuse of an entourage.
Before she could reach her refuge at Dragonstone, however, another disaster befell the Black party – King Aegon II himself launched a sudden attack and took the castle, although both him and his dragon Sunfyre were injured terribly in the fight. Rhaenyra arrived on Dragonstone only to walk right into her half-brother's trap.
Aegon II, even in the book, was never a nice guy (though immeasurably more badass than he is so far in the show).
Between death of his sister-wife Helaena, his dragon being near death, and himself crippled so badly that he could not walk on his own, whatever inclinations towards mercy Aegon II might have had disappeared. So he fed Rhaenyra to Sunfyre and made her son Aegon the Younger watch.
The war did not end at that, as Aegon II physical and mental trauma in turn led him on a downward spiral of insanity, until his short reign collapsed. Eventually above-mentioned Aegon the Younger ascended the throne as Aegon III, only to see deaths of the last of Targaryen dragons (not that he wasn't glad to see all the dragons gone, after what happened to his mother).