Maegor was the only son of Aegon the Conqueror by his sister-wife Visenya and the half-brother of Aenys I. Unlike his half-brother, Maegor was more like his father and grew up to become a strong warrior, being knighted by his father at age sixteen, the youngest at the time; even before then, Maegor was besting knights much older than he was. His mother gave him Dark Sister upon his knighthood. Unlike his brother, he refused to bond with any dragon other than Balerion, as he considered the Black Dread as the only dragon worthy of him, and waited for his father to die to bond with the beast. He intended to marry his niece Rhaena Targaryen, only for the High Septon to propose his niece Ceryse Hightower instead, but the marriage proved to be infertile, this caused him to seek other brides.
As Hand and as King
Upon his half-brother Aenys' ascension to the Iron Throne, the Faith of the Seven launched a major uprising against the Targaryen dynasty. Whilst they accepted Aegon on the throne due to his strength and his conversion to the Faith, they could not stomach the thought of his sons, born of incest, following him.
Maegor Targaryen finally mounted Balerion the Black Dread and shattered a rebellion in the Eyrie. For this, his brother named him Hand of the King and even gave him Blackfyre itself, the sword that once belonged to their royal father. This proved to be a good move on the king's part, as Aenys proved too weak and indecisive to handle the string of crises that awaited his reign. The reveal that he had wedded Alys Harroway while still married forced his brother to exile him to placate the Faith, leaving his wife Ceryse behind. In Pentos he met a courtesan named Tyanna.
When Aenys died in 42 AL, his mother Visenya brought him back from exile and Maegor took the throne (despite Aenys having a son). When Maegor faced off against Ser Damon Morrigen, the leader of the Warrior's Sons; they organised a trial of seven that ended with Maegor alone left standing. However, Maegor was savagely wounded, and comatose for days. When he awoke, he mounted Balerion and destroyed the Sept of Remembrance, massacring everyone inside it. This, and countless acts of savagery, violence and brutality earned him the name 'Maegor the Cruel'. When his nephew Aegon challenged him for the Throne, he killed Aegon and his dragon Quicksilver in a brief battle above the God's Eye.
Maegor's answer was to massacre those found rebelling against the crown, unleashing a wave of violence rarely seen in Westeros before or since. Tens of thousands, maybe more, died as Maegor set out to crush the rebels with every ounce of force he could muster. He accepted no parley and no pleas for mercy. He used the mighty Balerion to incinerate any rebel forces that opposed him.
When Maegor's second wife Alys gave birth to a deformed monstrosity, he executed her, her father Lucas Harroway, the bed maids, the Grand Maester (that had assisted the birth) and every member of House Harroway he could find. His marriage with Tyanna of Pentos proved to be just as barren as the first two.
During his reign Maegor began the construction of a new keep within the main castle complex in King's Landing, he hosted the builders a great feast, and shortly after executed all of them to ensure none of its secrets came out. Completed after his death, this fortress was dubbed Maegor's Holdfast. He also began the construction of the Dragonpit, but the massive fleeing of builders and stonemasons from the city forced him to use criminals from the dungeons, working under the supervision of builders from Tyrosh and Myr.
The passing of his mother Visenya in 44AC and the escape of Alyssa, Jaehaerys and Alysanne from Dragonstone angered him so much he killed his second nephew Viserys by torturing him to death for 9 nine days, leaving his body in the open to force Alyssa to come out. However, Alyssa did not appear. One by one, Maegor's lords started to withdraw their support from him.
The death of Ceryse made Maegor marry three women at the same time, all three of them were widowed by his actions and who's fertility was proven; Elinor Costayne and Jeyne Westerling, and his own niece Rhaena. Jeyne died giving birth to an eye-less, winged child and Elinor survived giving birth to two stillbirths. When Tyanna confessed she had poisoned the other brides, he killed her with Blackfyre and fed her heart to his dogs.
Jaehaerys support grew enormously, and most of his Bannermen, and two members of his Kingsguard, had abandoned him. The escape of Rhaena on Dreamfyre, who took her daughters and Blackfyre with her, left him a broken man.
Maegor died in 48 AL, allegedly bleeding to death after injuring himself on the Iron Throne. The smallfolk whispered that the throne itself had rejected and killed him for his crimes - other rumours imply that one of his Black Brides did the deed, that a vengeful builder of the Red Keep he'd failed to murder had done it, or that a member of his own Kingsguard had assassinated him. He was succeeded by Jaehaerys I Targaryen, Aenys' son, who made peace with the Faith and ended the war.
Character and appearance
Maegor was very tall, even taller than his father, but with a thicker, broader appearance, although this was due more to muscle than fat. He had short hair and a short beard closely matched to his jawline. He rarely laughed or smiled, and often looked angry, suspicious and sometimes paranoid. His entire reign as Hand and King was a period of warfare, and he took to wearing a breastplate covered by a surcoat prominently displaying the Targaryen symbol and colours to remind people of the authority of the ruling house. He inherited his father's sword and crown.
Maegor was a resolute, firm and commanding warrior who expected immediate and total obedience. Maegor was not a diplomat and did not believe in concessions or negotiations, only in the debates of steel and fire. He was merciless - after recovering from a coma induced by combat with the Faith Militant, Maegor's very first act was to obliterate the Sept of Remembrance and slaughter everyone in it, one of the myriad of conflicts he would have with the Faith Militant and their like. He had a dissolute side, taking multiple wives both within and outside his line, but had no children, all his wives giving birth to monstrosities and stillbirths. Only 2 of his six known wives survived him. The Targaryen tendency towards madness which emerged in later generations may have touched him, and before Aerys II Targaryen came along, Maegor was considered the epitome of a mad Targaryen.
An extremely cruel and violent man, combat was the first impulse to Maegor's mind - he would immediately respond to personal defiance by cutting down the man who had defied him. Maegor was one of the most fearsome warriors in Westeros in his day, and won the trial of Seven against Ser Damon Morrigen and his Warrior's Sons. In fact, Maegor was the singular survivor of that duel, though he took a savage blow to the head during it. Maegor was one of the youngest men to ever be knighted. Maegor's cruelty was such that he extinguished entire houses whimsically during his campaigns, and when he brought a thousand skulls back from one particular campaign, he claimed they belonged to defeated enemies but have popularly been theorised as unfortunate peasants who were in the wrong place at the wrong time.
Maegor Targaryen is credited well with being one of the more decisive kings in Westerosi history, almost as much as his father, and did not rely half as much on his Hand as other kings who succeeded him would. Bloody though his actions were, they were actions that he sanctioned and often carried out himself to the letter, whereas certain kings left ruling entirely to their Hand and indulged their own desires and fantasies in the meantime. Maegor was also extremely ambitious in his own right, determinedly building the Red Keep of King's Landing during his reign to house the Iron Throne and the royal family, and later the Dragonpit to house the dragons of House Targaryen. However, towards his death, Maegor proved to be increasingly impatient with other people, more so than he already was, and he rarely took to the fields when the realm actually started turning against him.
Maegor was widely the polar opposite of his father, even though they were both peerless warriors. While Aegon was capable of being ruthless, he was also willing to make peace with his enemies, whereas Maegor sought to shatter any rebellion, defiance or disagreement with him. Maegor craved violence, while Aegon did not, and Maegor was eventually hated by the people of Westeros - Aegon, at least, was loved and remembered as a great man who forged the foundations of a dynasty that his descendants would rule after him. Maegor, doing the opposite, wreaked havoc his entire reign and died childless.