- "Aegon the Unworthy legitimized all his bastards on his deathbed. Inevitably the greatest of these, Daemon Blackfyre, declared himself the rightful heir and ignited yet another war for the crown. After much bloodshed, Daemon was killed by an alliance of his brothers on the Redgrass Field."
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 the Redgrass Field.
- "Aegon the Unworthy legitimized all his bastards on his deathbed. Inevitably the greatest of these, Daemon Blackfyre, declared himself the rightful heir and ignited yet another war for the crown. After much bloodshed, Daemon was killed by an alliance of his brothers on the Redgrass Field. All the same, his descendants continued to threaten the Targaryens until half a century later, when Barristan the Bold slew Maelys the Monstrous on the Stepstones, extinguishing Daemon's line."
In the books
According to The World of Ice and Fire, House Blackfyre fled east to the Free Cities and continued to harass the Targaryen kings for decades. There were five Blackfyre Rebellions over the course of more than sixty years but of varying sizes. The first was also the largest and the one which very nearly succeeded due to the popularity of Daemon.
Historians alternatively call it the "First Blackfyre Rebellion", though of course, those who lived through the war only called it "the Blackfyre Rebellion" before the later rebellions occurred. By the time of the War of the Five Kings, many still refer to it as "the Blackfyre Rebellion" because it was by far the largest.
It is repeatedly said that the first Blackfyre Rebellion divided Westeros in half, though it appears that all of the Great Houses remained loyal to the Targaryens: rather, many of the major noble Houses in each of the Seven Kingdoms appear to have sided with the Blackfyres. It is stated that most of Daemon's supporters in the first rebellion were traditional foes of the Dornish - predominantly from the Reach and also from the Stormlands - who resented their recent union with the Iron Throne through Daeron I's marriage-alliance. Others who had some grudge or another with the ruling Targaryens also sided with the Blackfyres (Aegon IV had made numerous enemies), and still, others just opportunistically sided with the Blackfyres in hope of reward should they win.
The rebellion lasted about one year, and Daemon Blackfyre's success rose so high that at some point he even began to mint his own coinage. There may have been localized fighting throughout the Seven Kingdoms, but it appears that Daemon first consolidated his position with the armies of his main supporters, several major Houses in the Reach, and then led their armies in a campaign advancing northeast towards King's Landing. It is said that the fighting was heavy in the Riverlands (it is possible that Daemon looped north somewhat from the Reach into the Riverlands, then advanced east for King's Landing). Apart from this main thrust, Ser Quentyn Ball also won major Blackfyre victories in the Westerlands, killing Lord Lefford at the gates of Lannisport and then defeating Lord Damon Lannister. Daemon stood at the height of his fortunes by the Battle of the Redgrass Field - only a short distance from the capital city itself. The future of the realm hung in the balance during that climactic battle, as there was a very real chance that Daemon could have won it, in which case King's Landing would have soon fallen to the Blackfyres in the aftermath.
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, Aegon and Aemon, died in the first rebellion.
The "Second Blackfyre Rebellion" (212 AC) 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 Third Blackfyre Rebellion occurred in 219 AC, late in the reign of King Aerys I Targaryen. The invasion was led by Haegon I Blackfyre and Bittersteel. The conflict ended when at the end of the final battle Haegon was dishonorably killed after surrendering and giving up his sword. Bittersteel was captured and unwisely sent to the Wall, despite the urging of Lord Brynden Rivers and Prince Aerion Targaryen that he be executed. On the way to the Wall Bittersteel was freed by the Golden Company and returned to Tyrosh where he crowned Haegon's eldest son Daemon III.
The Fourth Blackfyre Rebellion occurred in 236 AC, and was led by Bittersteel and Daemon III, eldest son of Haegon I Blackfyre and grandson of Daemon I. The Blackfyres spent 15 years replenishing the ranks of the Golden Company then once again launched an invasion from 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 (260 AC), also known as the War of the Ninepenny Kings, was fought in the Stepstones and ended with the death of Maelys Blackfyre at the hands of Ser Barristan Selmy. Maelys's death also ended the male Blackfyre line.
One of the reasons that the Blackfyre Rebellions lasted for as long as they did is because, despite the failure of each subsequent rebellion, the Targaryens imposed very draconian punishments on anyone still suspected of Blackfyre sympathies - a short term solution which at the same time also kept anti-Targaryen resentment alive for some time. These harsh anti-Blackfyre measures were enforced by the vast spy network of Daemon's half-brother Brynden "Bloodraven" Rivers, who also served as Hand of the King for many years.
The Blackfyre Rebellions are somewhat analogous to the real-life Jacobite risings in the British Isles (1688-1746): both were a drawn out series of endemic rebellions in which a rival dynastic claimant repeatedly made failed attempts to seize the throne, after which the survivors fled overseas, spent years rebuilding their forces, then launched yet another invasion which also ended in failure, and the cycle continued. Similar to the harsh Targaryen treatment of Blackfyre sympathizers, the infamously brutal crackdowns hunting down anyone suspected of being Jacobite sympathizers are attributed with ironically inflaming anti-establishment sentiment, and keeping pro-Jacobite support alive for decades.
In "A Storm of Swords", Robb plans to issue a royal decree which legitimizes his half-brother Jon and makes him his successor. Catelyn objects (due to her personal feelings toward Jon), bitterly relating to the Blackfyre Rebellions. She points out that Aegon IV's foolish act of legitimizing his bastards caused so much pain, grief, war, and murder for five generations, until Selmy killed the last of the Blackfyre Pretenders, and claims that if Jon is legitimized - any future sons of Robb will never be safe. Robb dismisses his mother's objections, confident that Jon will never harm any of his descendants.