Maegor Targaryen

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.

Background
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.

Season 1
While visiting the Great Hall of the Red Keep and after being asked by Septa Mordane, Sansa Stark mentions Maegor the Cruel had the Red Keep built.

Season 2
Hallyne mentions Maegor as an advocate of the Alchemists' Guild and the originator of their work on the substance Wildfire.

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. He came to the throne ahead of Aenys's son Jaeharys in unknown circumstances. 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 and was succeeded by his nephew Jaeharys I.