Battle beneath the Gods Eye

"For the first time since the Doom of Valyria, dragon fought dragon in the sky. But Aegon's dragon was no match for Balerion the Dread, who was four times its size. When Aegon fell to his death, his army broke and fled. For slaying his own nephew, Maegor forever after became known as "the Cruel"."

- Varys

A battle took place beneath the Gods Eye between the forces of King Maegor Targaryen and Aegon Targaryen, Prince of Dragonstone.

Prelude
When King Aenys Targaryen died at Dragonstone, the exiled Prince Maegor was quick to return to Westeros and take the Iron Throne. Prince Aegon Targaryen, firstborn son of Aenys, had previously been named Prince of Dragonstone and heir to the throne. Early into his uncle's reign, Aegon raised an army of westermen and marched on King's Landing while Maegor was in Oldtown.

Battle
Before Aegon's forces could reach the capital, Maegor ordered his bannermen to swarm his nephew's army from all sides. As the forces of the Iron Throne came down on the rebel army, Maegor flew the dragon Balerion into battle. Prince Aegon, on his own dragon Quicksilver, was no match against Maegor. Balerion slew Quicksilver, and Aegon fell to his death. This crippled the morale of the rebel army, who broke and deserted.

Aftermath
King Maegor became known as "Maegor the Cruel" for killing his own nephew. Maegor ruled the Seven Kingdoms with an iron fist, and the Faith Militant uprising raged on.

In the books
In Fire & Blood, Aegon's army included Houses Charlton, Connington, Corbray, Dustin, Farman, Frey, Paege, Parren, Piper, Roote, Smallwood, Tarbeck, Vance, and Westerling. It was around 15,000 strong. Maegor's army included Houses Caswell, Harroway, Merryweather, Peake, and Tully, numbering around 5,000 men. Aegon lost around 2,000 men including Lords Tarbeck, Piper, Vance, and Farman, whereas Maegor 100 including Ser Davos Darklyn.