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. He was married to Jeyne Westerling. He died on the Iron Throne in unknown circumstances (in some versions of the tale, he was murdered by the Iron Throne) in unknown circumstances and was succeeded by his nephew, Jaeharys 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.
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