Balerion, called the Balerion the Black Dread, was one of Aegon I's three dragons that he used to conquer much of Westeros during the War of Conquest. Singers named him after an ancient god of the Valyrians, as they did with the other two dragons. He died at around 200 years of age, during the reign of Jaehaerys I.
Balerion is the largest of all the Targaryen dragons, and had jaws big enough to swallow a mammoth whole. According to song, the Iron Throne was forged using his fiery breath to melt the swords of Aegon's enemies. His fire was as black as his scales, his wingspan so vast that entire towns would fall under his shadow when he passed overhead.
During the battle that became known as the Field of Fire, the only time in history that all three of the fabled Targaryen dragons took to the sky at the same time. Between Vhaghar, Meraxes and Balerion, they killed four thousand men, burning them alive. King Mern died, with him House Gardener. The army was broken and Aegon I was victorious.