The Blackfyre Rebellion was an uprising led by Daemon Blackfyre, a legitimized bastard of King Aegon IV Targaryen and his cousin and lover Daena Targaryen, against his half-brother, King Daeron II Targaryen. The rebellion failed, and Daemon himself was killed in the Battle of Redgrass Field.
In the books Edit
House Blackfyre fled east to the Free Cities, and continued to harass the Targaryen kings for decades. There were as many as Five Blackfyre Rebellions over the course of sixty years, but of varying size. The first was also the largest, and the one which very nearly succeeded due to the popularity of Daemon.
Daemon had many children, including seven sons and possibly several daughters, which allowed the Blackfyre line to survive through multiple failed rebellions. Daemon's eldest two sons died in the first rebellion.
The "Second Blackfyre Rebellion" (212 AL) was little more than a plot which was rooted out before it could achieve fruition. Daemon II Blackfyre ended up being taken alive as a prisoner, so that Daemon's fourth son Haegon couldn't be crowned king in exile.
The Fourth Blackfyre Rebellion occurred in 236 AL, and was led by Bittersteel and Daemon III, grandson of Daemon I. The Blackfyres were able to assemble an army in the Free Cities, which landed at Massey's Hook in the southeast of the Crownlands (near the northern border of the Stormlands). Daemon III had depended on the hope that numerous major Houses in the Seven Kingdoms would switch over to his side after he won a few victories on mainland Westeros. Unfortunately, his anticipated groundswell of support never occurred: Daemon I had been dead for forty years by this point, and the once fervent support for the Blackfyre cause had been forgotten. To many lords, Daemon III's army seemed like just a collection of ragged mercenaries carrying tattered old banners. Ser Duncan the Tall emerged as one of the great heroes of the conflict, when he personally slew Daemon III at the Battle of Wendwater Bridge, ending the war.
The fifth and final Blackfyre Rebellion (259-261 AL) was known as the War of the Ninepenny Kings.