And you thought Red Wedding was bad enough…
Lucerys Velarion's death at the end of House of the Dragon Season 1 Episode 10 not only was the final spark that started the civil war for real, but set up what is almost certainly going to be the most horrifying scene in the entire series, known among the fans of George Martin's Fire & Blood (and previous versions of the same story) as the Blood and Cheese incident.
After the news of Lucerys' death reach the rest of the Blacks, they decide to take revenge through assassination.
Specifics, as is common for the book, which is framed as a historical account, written long after the fact, are vague, but, as it appears, Daemon Targaryen ordered the hit, and Mysaria found suitable men for the job. These men are only named by the historian as Blood (a monstrously strong former City Watch sergeant) and Cheese (a ratcatcher, who knew the hidden tunnels of the Red Keep). It is mentioned that they might have been initially targeting King Aegon II, but he was too well-guarded.
So instead they've snuck into women chambers, capturing Alicent, Queen Helaena and her young children. Then they forced Helaena Targaryen to choose which of her sons, Jaehaerys or Maelor, will die. "An eye for an eye, a son for a son," as they allegedly said, threatening to kill everyone, if she does not choose.
Helaena chose Maelor as the younger son, less likely to understand; Blood instead killed Jaehaerys, fleeing with his head when Helaena screamed, alerting the guards outside.
Later he was captured trying to smuggle the prince's heads to Harrenhall so that he could collect his reward from Daemon, and under torture revealed whatever he knew about the plot. Cheese got away, so King Aegon II ordered to hang every ratcatcher in King's Landing his guards could find instead.
Fire & Blood features a lot of grisly scenes, which are likely to make their way into the series, from mass slaughter to people getting eaten alive by dragons, but pretty much everyone agrees that this was the most horrifying of them all, visceral even in the relatively dry "history book" telling and involving innocent children.
One wonders when exactly in Season 2 it is going to take place. The book goes to it straight after Lucerys' death, but given its weight and impact on other characters, the series' writers might use it as a turning point, after which the war grows truly bitter, closer to the middle of the season.