The Battle of the Redgrass Field was a military engagement fought approximately a century before the events of Game of Thrones. It was the final battle of the Blackfyre Rebellion, where Daemon Blackfyre was slain and his attempt to unseat his half-brother King Daeron II Targaryen was defeated.
Daemon Blackfyre, legitimized bastard son of King Aegon IV Targaryen, led a massive rebellion against his half-brother King Daeron II Targaryen. The climax of the rebellion was the massive Battle of Redgrass Field, fought near King's Landing.
In the books
This battle took place in approx. 195 or 196 AL and was the concluding battle of the Blackfyre Rebellion. Daemon Blackfyre, the bastard son of King Aegon IV Targaryen by his cousin Daena, claimed that his half-brother Daeron II Targaryen was illegitimate, being born of a liaison between his mother and her other brother. This would give him, as Aegon IV's next-eldest and legitimized child, the right to the Iron Throne. Daeron II disputed this and the resulting civil war lasted more than a year, with the realm divided between the red dragon (the Targaryens) and the black (the Blackfyres).
The decisive battle came on a wide, nondescript field southwest of King's Landing. Daemon Blackfyre's forces gained the upper hand over the royalist army, with Daemon defeating Ser Gwayne Corbray of the Kingsguard in single combat. While chivalrously dismounting to help his wounded foe from the battlefield, Daemon was slain by a volley of arrows fired by the Raven's Teeth, an elite archery formation led by his half-brother Brynden Rivers, popularly known as "Bloodraven". The Raven's Teeth caused immense damage to Daemon's army, slaying two of his sons as well. Daemon's other bastard half-brother, Aegor Rivers, known as "Bittersteel", led a cavalry charge which routed the Raven's Teeth and resulted in a duel between himself and Bloodraven that ended with Bloodraven losing an eye. The rest of the Targaryen army, commanded by Prince Maekar formed a shield wall which managed to hold out against the mad attack of Bittersteel's forces. However, Bloodraven's gambit to distract Bittersteel allowed Prince Baelor "Breakspear" Targaryen to arrive with substantial reinforcements and attack the committed Blackfyre army in the rear. Caught between the hammer of Baelor's reinforcements and the anvil of Maekar's shield wall, the Blackfyre army was destroyed. The number of deaths that day - more than ten thousand - resulted in the grass turning red from blood, giving the battle its name.
Redgrass Field was one of the largest battles fought in Westerosi history, and the largest fought in the eighty years between the First Blackfyre Rebellion and Robert's Rebellion.
Bittersteel was able to flee the field with a loyal core of supporters, who subsequently fled across the Narrow Sea to the Free Cities, along with Daemon's younger sons. They later founded the Golden Company, a famous mercenary army, in order to keep some semblance of a standing army. The Blackfyres attempted several more times to invade Westeros until being finally defeated fifty years later in the Fifth Blackfyre Rebellion, known as the "War of the Ninepenny Kings".