Like Aegon IV Targaryen and Baelor I Targaryen, Maegor does not physically appear in any of the ASOIAF media (this includes Game of Thrones), and is only mentioned in A Game of Thrones and A Storm of Swords, as well as several of the Dunk and Egg novellas.
|“||Now people name Maegor "the Cruel", but I doubt any dared in his day. His strength was all too rare in the degenerate Targaryen blood.||„|
|~ Joffrey Baratheon in "The Red Keep", Histories & Lores|
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. 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.
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.
Aenys I proved too weak and indecisive to handle the crisis and appointed Maegor as the Hand of the King, with responsibility for putting down the rebellion. 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 Aenys died in 42 AL, Maegor took the throne, despite Aenys having sons. When a Grand Maester pointed out Aenys' sons should inherit, Maegor decapitated him with a single blow. Maegor later slew Aenys' eldest son, and tortured another of his nephews to death. He continued the oppression wherever he could, earning him the name 'Maegor the Cruel'.
Eventually, Aenys' last son, Jaehaerys, rebelled against his uncle, winning the support of Lord Robar Baratheon. The rebellion spread, with even two Kingsguard leaving Maegor. The King was later found dead on the Iron Throne, covered in blood. His death is unknown, but many believe he killed himself using the blades of the Iron Throne.
During his reign Maegor began the construction of a new keep within the main castle complex in King's Landing. Completed after his death, this fortressed was dubbed Maegor's Holdfast.
Maegor is one of the Targaryen kings to historically suffer from "Targaryen madness". He was often seen as cruel, never treating animals well and keeping to himself. He enjoyed war and battle, but it was violence he most craved, and death and absolute mastery over all he deemed his. He is considered as the perhaps the earliest fearsome tyrant of Westros, much earlier than the later Mad Targaryens.
Maegor also became the proverbial tyrannical king to the people of the Seven Kingdoms: any time another potential ruler appeared brutal or tyrannical, it was whispered that they would be "the next Maegor the Cruel". This only stopped in the last generation, when Aerys II "the Mad King" terrorized the realm, ultimately leading to his downfall in Robert's Rebellion - after which the proverb shifted to saying "he would be the next Mad King", or when Tyrion remarks that Joffrey is on his way to becoming "Aerys the Third".