In Season 5, Episode 10 of Game of Thrones, Shireen Baratheon was shown reading a big book called Dance of the Dragons, which is an actual book written by George RR Martin, inspired by which the HBO superhit House of the Dragon is created. In the scene, Stannis Baratheon walks into his daughter’s bedroom, finds her reading the book, and tells her about it. This scene is iconic, as it was the last conversation Stannis would have with his daughter before she was burned alive as a sacrifice to the Fire God Lady Melisandre prays to.
Speaking of the book, Shireen says, “It’s the story of Rhaenyra Targaryen and her half-brother Aegon for control of the seven kingdoms. Both thought they belonged on the Iron Throne. When people started declaring for one or the other, their fight split the realms in two,” Shireen continues, “Brothers fought brothers, dragons fought dragons. By the end, thousands had died and it was a disaster for the Targaryens as well. They never really recovered.
Based on this, let’s explore what led to the House of the Dragon’s demise.
The realms have been split into two: Rhaenyra side and Alicent side

Alicent Hightower and her family
After Episode 8 of House of the Dragon, we now know that Alicent thinks her son Aegon is the heir, and Rhaenyra thinks otherwise. This will most likely result in the formation of two camps, one led by Otto Hightower with his daughter and grandson (King) as puppets. On the other side, you have Rhaenyra and her family. Few houses would side with Rhaenyra; however, she has more dragons by her side than the Hightowers.
Shireen says exactly this: “When people started declaring for one or the other, their fight split the realms in two,” which means we are very close to seeing that happen.
Symbolism of Shireen Reading Dance of the Dragons before her death

Shireen talks about the dragon dance to Stannis
During the Dance of the Dragons, family members clashed. In the book (MAJOR SPOILER ALERT), Rhaenyra had to sell her crown to escape Bravos and return to Dragonstone in her final days. Her half-brother, Aegon II Targaryen, kidnapped her and gave her to his wounded dragon, Sunfyre. As a result, Rhaenyra was murdered and burned with fire. Aegon the Younger, son of Rhaenyra, was forced to witness and hate dragons for the rest of his life.
The symbolism here is that Shireen was also burned alive and her mother, Lady Selyse Baratheon, was forced to watch. After witnessing his death, Selyse also ended up committing suicide by hanging.
How House Targaryen Was Destroyed
In ancient Valyria there were thousands of dragons. Fast forward to 126 AC (the current time of House of the Dragon), and we only have seventeen, most of which have yet to be shown in House of the Dragon. We know that after the civil war is over and Aerys Targaryen, the mad king is born, all the dragons will be dead, that’s why the mad king used wildfire, and we don’t see any dragons in Game of Thrones until Danaerys hatches the three dragon eggs. Therefore, it is the “Dance of the Dragons” or, literally, the fight between dragons that destroys House Targaryen, and this is precisely what King Viserys said to Rhaenyra; only the House of the Dragon can destroy itself.