Who is Vhagar? House of the Dragon's newest dragon explained

gamesradar.com:

House of the Dragon episode 6 introduces a new dragon to the mix – Laena Velaryon rides an absolutely massive creature that we haven't seen in the show before. This dragon is none other than Vhagar, the oldest and biggest of the dragons currently alive in Westeros. This fire-breather has a particularly epic history, as detailed in the works of George R.R. Martin. Now, before we go any further, the following is totally spoiler free, so you can read up on Vhagar whether you're up to date or not. 

Vhagar was hatched on Dragonstone in what's known as the Century of Blood, which spans the hundred years or so before Aegon the Conqueror and his two sisters took Westeros. The dragon was claimed by one of those sisters: Visenya Targaryen, Aegon's older sister – and, later, his wife, along with their other sister Rhaenys (that's the Targaryens, for you). Aegon flew Balerion and Rhaenys rode Meraxes: Vhagar is the only one of the three dragons still alive. 

Visenya flew Vhagar throughout Aegon's Conquest, including in one huge battle that killed four thousand people when all three Targaryen dragons took to the skies. Vhagar even helped make a peaceful surrender with House Arryn possible: when Visenya arrived at the Vale atop her dragon, the young King Ronnel was captivated by Vhagar and asked Dowager Queen Sharra Arryn, his mother, if he could ride the great creature. Sharra surrendered to Visenya, and Ronnel got his dragon ride. 

Aegon, Visenya, and Rhaenys later went to war with Dorne – the only kingdom not captured in the Conquest. Rhaenys and Meraxes were killed in the conflict, and in retaliation, Aegon and Visenya unleashed hell on Dorne in a period known as The Dragon's Wroth. Ominous. 

When Aegon the Conqueror died, Vhagar