Vhagar was one of the dragons used by Aegon The Conqueror and his sisters during The War of Conquest. It was named for one of the gods of old Valyria. Vhagar was ridden by Visenya Targaryen.
During the battle that became known as the Field of Fire, for the only time in history all three of the original Targaryen dragons took the sky at the same time. Together, Vhagar, Meraxes, and Balerion the Black Dread killed four thousand men, burning them alive. King Mern died, and with him House Gardener. The army was broken and Aegon I was victorious and during that battle the fire of Vhagar could melt a knight's armor and cook him inside. During The Dance of Dragons Vhagar was ridden by Aemond Targaryen.
The dragon was already growing to be nearly as big as Balerion, and only Vermithor, the Bronze Fury, was close to its size. It was the biggest dragon alive during the Dance.
The huge dragon killed Prince Lucerys Velaryon and his dragon Arrax, that was 5 times smaller, during the dance of Shipwrecker Bay. Despite Arrax being faster, a terrible storm that day caused him to slow down alott, and both dragon and rider were easy prey for Aemond and Vhagar.
Next Vhagar and Aemond worked together with Aegon II and his dragon Sunfyre, the Golden, to kill Princess Rhaenys Targaryen, the Queen who Never Was, and her dragon Meley, the Red Queen, during the Battle of Rook's Nest. While Meley could have defeated Vhagar alone, she couldn't defeat both dragons and was killed alongside her rider. After the fight, only Vhagar remained relatively unharmed, its partner Sunfyre was greatly injured, alongside King Aegon II.
Aemond took Vhagar to ravage the Riverlands alongside Lord Commander Ser Criston Cole, but Aemond left the army without a dragon as he wanted to cause destruction on his own, this in the end provoked the death of Cole. Dragon and rider remained destroying the Riverlands for many months until they were challenged by Prince Daemon Targaryen and his dragon Caraxes, the Blood Wyrm, to a fight in Harrenhal, as Daemon wanted revenge for the murder of his stepson Lucerys.
They waited for thirteen days at the shores of the God's Eye, until Aemond and Vhagar arrived at the fourteenth. During the fight, Daemon jumped from Caraxes in mid air, and killed Aemond by stabbing him with Dark Sister in the eye. Both dragons crashed in mid air and fell to the lake, Vhagar drowned, Caraxes managed to swim ashore, and died sometime later, the body of Daemon was never found. The bodies of Vhagar and his rider weren't recovered until some years after the war, the body of Aemond was still strapped to his chair, with Dark Sister in his eye.
After Vhagar died its skull can be found at the Red Keep in King's Landing.