Balerion, called the Black Dread, was one of the three great dragons of Aegon the Conqueror and his sisters, that were used to conquer much of Westeros during the War of Conquest. Balerion was killed some time during the reign of Jaehaerys I, around 200 years of age.
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.
Balerion was named so after the ancient god of Valyrian Freehold he gained recognition as the 'Black Dread' during the War of Conquest, when Aegon rode him to victory and conquered Westeros. He became notorious in 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.