Vhagar was the dragon ridden by Visenya Targaryen during the War of Conquest. Though smallest of the dragons, Vhagar was still large enough to swallow a horse whole, and its flames could melt armor.
Vhagar was flown by Aegon's sister-wife Visenya during the War of Conquest. She was one of the three dragons present at the Field of Fire, the decisive battle of the conquest that ensured Targaryen mastery of the Seven Kingdoms.
Nonetheless, House Arryn thought that the Vale could still resist Targaryen domination by hiding behind the impregnable Mountains of the Moon along their borders, and fortifying the mountain passes beyond any hope of attack. Similar to how Aegon had dealt with Harrenhal, Visenya simply flew Vhagar directly over the mountains, and came to rest in the courtyard of the Arryn's own castle, the Eyrie. At the time, the King of Mountain and Vale was only a small boy, Ronnel Arryn, whose mother Sharra was acting as his regent. Visenya did not need to burn the Eyrie as Aegon had burned Harrenhal: her display of force with Vhagar was enough to compel the Arryns to surrender. Young Ronnel did ask one condition for his surrender: in return, Visenya would give him a ride around the Eyrie atop her dragon. The Vale surrendered, and true to her word, Visenya gave Ronnel a ride atop Vhagar.
Vhagar lived for another 130 years after the Conquest, until the great Targaryen civil war known as the Dance of the Dragons. In The Dance of the Dragons, A True Telling, Grand Maester Munkun wrote that Ser Byron Swann wanted to kill the dragon Vhagar so he polished his shield for a week so that it was like a mirror and he crouched behind it and crept forward, hoping the dragon would only see its own reflection, but the dragon only saw a dumb man holding a mirrored shield and burnt him to a crisp.
In the BooksEdit
Laena Velaryon, daughter of Lord Corlys Velaryon and Princess Rhaenys Targaryen and wife to Prince Daemon Targaryen, rode Vhagar during the reign of Viserys I Targaryen. Laena eventually died in childbirth and not long after, Vhagar accepted Prince Aemond Targaryen as his rider.
After Meraxes and then Balerion died, Vhagar was the last of the original three Targaryen dragons. Meraxes died fighting in Dorne, while Balerion later died of old age (about forty years before the Dance of the Dragons).
Vhagar was ridden by Aemond Targaryen during the Dance of the Dragons, the Targaryen civil war between Aegon II and Rhaenyra Targaryen. Aemond sided with his brother Aegon II. Vhagar was the greatest and most powerful living dragon during the Dance, by that time grown nearly as big as Balerion was during the War of Conquest, and she was one of the major advantages possessed by Aegon II's faction. In one of the opening moves of the war, Aemond rode Vhagar to attack and kill Rhaenyra's son Lucerys at Storm's End, while he was riding the young dragon Arrax - Vhagar killed both with little difficulty. At the Battle of Rook's Rest, Aegon II's forced laid a trap for Rhaenys Targaryen and her dragon Meleys (not the original Rhaenys, but her grandson Jaehaerys I's grandaughter). After Rhaenys arrived, she was ambushed by Aemond riding Vhagar and Aegon II riding Sunfyre. While Sunfyre was badly wounded, the great dragon Meleys was killed. Afterwards, Aemond used Vhagar to burn out vast swaths of the Riverlands, which had sided with Rhaenyra. Nonetheless, Vhagar and Aemond were eventually killed during the Dance, in a great duel with Prince Daemon Targaryen and his dragon Caraxes at Harrenhal.
Grand Maester Munkun's historical account claims that Ser Byron Swann attempted to sneak up on and kill Vhagar by hiding behind a mirrored shield - which is mentioned in Season 5 of the TV series. In the novels, Swann used this tactic in imitation of the legendary tale in which Serwyn of the Mirror Shield snuck up on and killed the dragon Urrax by hiding behind a polished shield, so the dragon only saw its reflection (it isn't clear if Serwyn and Urrax ever really existed). The TV episode didn't have time to explain this in more context, but at one point Tyrion remarks that Munkun's account was inaccurate on this point: Balon Swann actually tried to sneak up on Rhaenyra's dragon Syrax, not Vhagar. House Swann was sworn to the Baratheons, who declared for Aegon II, so as Tyrion pointed out, it wouldn't make sense for Swann to try to kill the greatest dragon belonging to Aegon II's faction, ridden by Aegon Ii's own brother. Either way, of course, the result was the same: the legendary trick didn't actually work and the dragon killed Swann (whether because real dragons can actually recognize their own reflection, or because the dragon could obviously tell that there was a man behind it, is unclear).
She fought in more major Westerosi wars than any other Targaryen dragon: the War of Conquest, the Faith Militant uprising, and the Dance of the Dragons. Meraxes only fought in the War of Conquest. Balerion did fight in the Faith Militant uprising (ridden by Visenya's own son Maegor after Aegon died), but he died before the Dance of the Dragons - though Balerion did fight in previous wars outside of Westeros, such as the "Century of Blood" in the Free Cities.
Vhagar was the mother of several of the second-generation Targaryen dragons, but exactly which ones have not yet been specified.
Meraxes · Morghul · Shrykos · Sunfyre · Syrax · Tyraxes · Valryon · Vhagar