The universe created by George Martin in 'A Song of Ice and Fire' seems to know no bounds — and that is why every year the HBO studio receives more and more offers for spin-offs and prequels, even almost 6 years after the end of Game of Thrones. So, last year, fans of the cinematic universe received the second season of House of the Dragon, and this year a spin-off called A Knight of the Seven Kingdoms will start — the series will tell about the travels of the knight Duncan the Tall and his squire Egg, who will later become King Aegon V Targaryen. However, to the delight of fans of the Targaryen family, the plot will not focus only on the story of Aegon — the series will also include the one who played a key role in the fall of the most powerful dynasty of Westeros.
Aerion Targaryen will appear in the future series — he was the one who started the “madness” of the entire family

Prince Aerion was the son of Maekar I Targaryen and Lady Dianna Dayne, and the brother of Aegon Targaryen, who will be the focus of A Knight of the Seven Kingdoms. As the king's second in line, Aerion knew that he would never be destined to sit on the Iron Throne, although he clearly dreamed of it. However, the prince seemed to have come to terms with his position as second in line and tried to live a better life, using all the Targaryen wealth to fulfill his whims, but in fact, hiding his evil nature under luxurious jewelry and elegant clothes.
As George R.R. Martin wrote, Aerion repeatedly threatened his older brother with violence and frightened everyone around him with his belief that he was, in fact, a dragon in a man's body. It was Aerion who was one of the first to confirm that the Targaryens were not so mentally healthy — later, members of the dynasty would be perceived as crazy, and Aerys II Targaryen, Daenerys' father, would even be given the nickname the Mad King.
Aerion's fate is a spoiler for what happened to the entire Targaryen dynasty
Although, as George Martin claims, the people of Westeros were glad that the character essentially excluded himself from the contenders for the Iron Throne (after getting drunk with friends, Aerion decided that a cup with wildfire inside would help him become a dragon, and died, burned from the inside), they had no idea that Aerion would forever determine the fate of his entire family. During the dynastic crisis that occurred after the hero's death, Aerion's son was not even considered as a potential candidate, believing that he could inherit the violent temper and psychological problems of his father.