WARNING! This article contains SPOILERS for House of the Dragon season 1!skull of Balerion the Black Dread, but leaves out the third dragon from Aegon’s Conquest. Since King Aegon I Targaryen’s Balerion the Black Dread died before Viserys sat the Iron Throne, the oldest and largest creature during House of the Dragon is Vhagar, who is now ridden by Aemond Targaryen. Vhagar is the only remaining dragon ridden by the Targaryens during Aegon’s Conquest, having originally bonded with King Aegon’s sister-wife Visenya. The third dragon represented in House Targaryen’s three-headed dragon sigil is Meraxes, who was ridden by Aegon the Conqueror’s other sister-wife Rhaenys.
Aegon and his two sisters conquered Westeros together on dragonback; Aegon on Balerion, Visenya on Vhagar, and Rhaenys on Meraxes. While Aegon and Visenya would rule Westeros for decades after the Conquest, their sister Rhaenys was subject to an early tragedy. Ten years into House Targaryen’s long reign, Rhaenys led an assault on Hellholt during the First Dornish War. However, the Dornish were prepared and launched an iron bolt from a scorpion upon her arrival, which impaled Meraxes’ eye and sent Rhaenys and her dragon falling from the sky. As Meraxes fell to her death, House of the Dragon’s legends say that Rhaenys either fatally fell from her seat or was crushed to death beneath her dragon.
How Powerful Was Meraxes Compared To Balerion & Vhagar?
Balerion the Black Dread is still regarded as the biggest and most powerful dragon in Westerosi history, with Vhagar now being the strongest during House of the Dragon’s story. Meraxes was thought to be older and larger than Vhagar, suggesting the former was the more powerful dragon during Aegon’s Conquest. However, since Meraxes died somewhere between the ages of 62 and 124 years old, Aemond's nearly 200-year-old dragon Vhagar is likely more powerful during House of the Dragon than the mount of Princess Rhaenys' namesake at the time of her death. Dragons are implied to never stop growing, so Vhagar’s power presumably sured Meraxes a few decades after the latter’s demise.
How Aegon The Conqueror Reacted To Meraxes & Rhaenys' Deaths
The deaths of Rhaenys and Meraxes were detrimental to both King Aegon I Targaryen and Queen Visenya, leading to a two-year period of destruction in Dorne known as the “Dragon’s Wroth.” The grieving Aegon and Visenya reacted to their sister’s death by burning every castle and holdfast in Dorne except for Sunspear, as well as ordering bounties on Dornish lords. Aegon the Conqueror married Rhaenys out of desire and Visenya out of duty, which is part of why his response to the former’s death was so devastating.
While Rhaenys’ remains were never returned to Aegon the Conqueror, the Dornish eventually brought the skull of Meraxes to King’s Landing as a peace offering. According to Fire & Blood, Meraxes’ skull was then mounted on the wall of the Red Keep’s throne room, and was seemingly kept there until Robert Baratheon usurped the Iron Throne. However, House of the Dragon hasn’t kept this detail, as it appears dragon skulls have been kept elsewhere. As such, it’s possible that House of the Dragon will depict Meraxes’ skull in a cellar later on.