Maegor I Targaryen, full name Maegor of House Targaryen, the First of His Name, called Maegor the Cruel, is an unseen character in the second season. He died before the time of the series, and is not expected to appear. He was a King of Westeros in the Targaryen dynasty.
Maegor was a ruler in the Targaryen dynasty. He had the Red Keep built during his reign, including Maegor's Holdfast, which was named after him. The holdfast is the tower at the center of the Red Keep in King's Landing.
In the books
In the A Song of Ice and Fire novels, Maegor is the second son of Aegon I Targaryen. His mother was Aegon's sister wife Visenya Targaryen. He served as Hand of the King to his older half-brother Aenys I Targaryen during his rule. Indeed, Aenys was such a weak ruler that Maegor was always considered the true power behind the throne. He came to the throne ahead of Aenys's son Jaeharys in unknown circumstances, possibly using his authority as Hand of the King to outright usurp the throne. His reign coincided with the uprising of the Faith Militant and he earned his nickname for offering a bounty on the scalps of members of the order. Maegor completed construction of the Red Keep and gave his name to Maegor's Holdfast. He had the builders executed to keep the secrets of the hidden passages he had included. He also executed at least three Grand Maesters during his reign.
An only child, Maegor initially married one of his half-sisters, daughters of his aunt Rhaenys Targaryen (and a full sister of Aenys). Aenys himself had chosen not to continue the Valyrian custom of incestuous marriage. At some point Maegor's first wife died, and he remarried to a woman named Jeyne Westerling. Three centuries later, Robb Stark also married a woman named Jeyne Westerling (changed to Talisa Maegyr in the TV series), but she was just another woman from House Westerling, and it is a coincidence that one of her family members three hundred years earlier had the same name. Maegor was actually married to eight wives during his lifetime, two of them his half-sisters, but he kept killing them because they could not produce heirs.
When the two brothers were young, Aenys bonded with the dragon Quicksilver, one of the progeny of Aegon I's original three dragons. Maegor, however, refused to bond with any dragon, claiming none were worthy - or perhaps none of the available ones at the time. After their father died, Maegor ended up bonding with Balerion the Black Dread himself, Aegon I's own dragon. Maegor would later use Balerion to devastating effect during the Faith Militant uprisings.
Meagor was a preternaturally skilled swordsman (not unlike the later Jaime Lannister), at the age of barely 12 he easily defeated squires five years older than himself. He was one of the youngest men ever knighted, at only 16 years old. When Aenys succeeded to the Iron Throne upon their father's death, he presented Maegor with Aegon I's Valyrian steel sword Blackfyre, admitting that Maegor was always much more of a warrior than he ever was.
Maegor eventually died on the Iron Throne in unknown circumstances: in some versions of the tale, he was murdered by the Iron Throne, while others suspect he was poisoned. As Maegor died without issue, he was succeeded by his nephew, Aenys' son Jaehaerys I.
While Maegor was an infamously ruthless man, he was not necessarily considered to be "insane", the way that many later Targaryens became insane due to generations of incestously marrying brother to sister to "keep the bloodlines pure". History remembers Maegor as a cruel man, but he was perhaps a brutal man well-suited to a brutal time, when the early Targaryen kings were still insecure in their position and surrounded by enemies. Whatever the case, he became remembered as a controversial figure, commonly recalled as "Maegor the Cruel", and later generations of the Targaryen dynasty were tactful enough never to name one of their sons "Maegor" again.
Kings of the Seven Kingdoms
Aegon I, the Conqueror · Aenys I · Maegor I, the Cruel · Jaehaerys I, the Conciliator · Viserys I · Aegon II · Aegon III, the Dragonbane · Daeron I, the Young Dragon · Baelor I, the Blessed · Viserys II · Aegon IV, the Unworthy · Daeron II, the Good · Aerys I · Maekar I · Aegon V, the Unlikely · Aerys II, the Mad