She thinks he's talking about their son Aegon (technically Aegon II) instead. Alicent, who has never heard of this because this sacred knowledge is only reserved for Targaryen heirs, misunderstands Viserys' words because she's missing crucial context.
He starts responding to Rhaenyra's question from the other night, about whether he really believes in the Song of Ice and Fire, a secret prophecy their ancestor Aegon the Conqueror had about the fate of the world. In his last moments, Viserys, in pain and delirious in bed, confuses his wife, Alicent, for his daughter, Rhaenyra. While he declared his daughter Princess Rhaenyra Targaryen as his heir years ago, he just accidentally imparted his wife, Alicent Hightower, with intel that could favor their son as heir instead. Moments before his death, as he discusses the famed Song of Ice and Fire prophecy, he unknowingly further complicates the battle for his succession. In House of the Dragon episode 8, Paddy Considine's King Viserys Targaryen makes a tearful exit but sets in motion crucial events in the Targaryen civil war, known as the Dance of Dragons.
