Jaehaerys I Targaryen

"We're west of Castle Black, but the Nightfort's closest to us. It was the first castle on the Wall. The Watch abandoned it during the reign of King Jaehaerys the First."

- Samwell Tarly to Gilly

Jaehaerys I Targaryen is an unseen character in Game of Thrones. He is not expected to appear in the series, being long dead by the time it begins.

Background
Jaehaerys I was king of the Seven Kingdoms, the fourth king of the Targaryen dynasty, and reigned two centuries before the War of the Five Kings.

Season 3
Trying to make his way back south of the Wall, Samwell Tarly explains to Gilly that there is a secret passage at the Nightfort, the first castle on the Wall, and that the castle was abandoned during the reign of King Jaehaerys the First.

In the books
In the A Song of Ice and Fire novels, Jaehaerys I is fondly remembered as one of the best kings the Targaryen dynasty ever produced, and his long reign became synonymous with peace and prosperity. He was the longest-ruling king in history, sitting on the Iron Throne for fifty-five years, and due to his advanced age by the end of his reign is often called "the Old King". He is also known as "Jaehaerys the Conciliator" because he brought an end to the eleven year uprising by the Faith of the Seven which had been fought by his uncle Maegor the Cruel. Jaehaerys offered the Faith generous terms of amnesty, so long as they acknowledged Targaryen rule and disbanded the military orders of the religion, known as the Faith Militant.

Jaehaerys I was the son of Aegon I's older son, Aenys I, a weakling who did not easily hold onto the throne. Because of the friction that the Targaryens' incestuous marriage practices caused with the Faith of the Seven, Aenys married outside of the family (despite having several sisters) in an attempt to placate the Faith. Aenys married Alyssa of House Velaryon - a Valyrian family who had followed their Targaryen liege-lords by settling on the islands in Blackwater Bay. Ultimately this failed to appease the Faith because, fundamentally, Aenys and Maegor were already the product of incest, and who each of them subsequently married wouldn't change that.

During Aenys's reign his younger half-brother Maegor served as Hand of the King and was the real power behind the throne. Through unspecified events Maegor somehow succeeded his brother ahead of his nephew Jaehaerys, possibly through openly seizing the throne in a coup after secretly holding the real power for so many years. Maegor eventually died childless, however, which allowed Jaehaerys I to succeed him.

Following Targaryen custom, Jaehaerys married his own sister Alysanne. She was actively involved in court affairs and became one of Jaehaerys's chief counselors, if not practical co-ruler. Alysanne also rode her own dragon, named Silverwing. She is also fondly remembered in the Seven Kingdoms, as "Good Queen Alysanne".

Jaehaerys and Alysanne had nine children together. Their first born son was named Aemon, and their second born son was named Baelon. They also had at least two daughters, Daella and Saera. Aemon married outside of the family, to Jocelyn Baratheon, and had a daughter whom he named Rhaenys, after his father's grandmother. Aemon died in 92 AL, but Jaehaerys chose Baelon over Rhaenys as his heir - despite the fact that an older daughter normally succeeded ahead of a younger uncle according to the normal inheritance laws in Westeros. Baelon himself had two sons, Viserys and Daemon, and also was named Hand of the King, but later died in 101 AL, two years before Jaehaerys I died, leading to a Great Council to determine who the heir should be. Rhaenys again put forward her claim, along with those of her children Laenor and Laena Velaryon, but she was again rejected in favor of Jaehaerys's grandson Viserys I Targaryen. Daella married Lord Arryn and had a daughter, Aemma Arryn, who was the first wife of Viserys I and the mother of Rhaenyra Targaryen. Saera left across the Narrow Sea and did not return from the East, greatly saddening Jaehaerys I in his old age.

The Nightfort was not simply abandoned during Jaehaerys I's reign, but actually visited by him and Queen Alysanne. The royal couple had made a formal visit to Winterfell in a great flight consisting of six dragons. During the visit to Winterfell Alysanne grew bored, and on a whim flew north to the Wall on her dragon Silverwing (when Samwell Tarly passed through the Nightfort over two hundred years later, his mind boggled that a live dragon once stood in its courtyard). She was followed later by Jaehaerys himself. The Night's Watch had been in a serious decline for several decades, however, because ever since the Targaryen Conquest the Iron Throne had put an end to the constant petty wars which had been going on between the Seven Kingdoms for centuries. While this greatly benefited the prosperity of the new unified realm, it meant that the constant trickle of recruits from the losing sides of those petty wars dried up. Alysanne was deeply impressed with the bravery of the men of the Night's Watch, and convinced her husband to double the size of the lands of the Gift. The boost of support that the New Gift gave the Watch helped stall its decline for several generations (though it was in full decline again two hundred years later when Jon and Sam joined it). At the same time, the royal couple recognized that the Watch could no longer adequately man the Nightfort, and convinced them to abandon it. A new, smaller castle was built east of the Nightfort, named Deep Lake, to replace its guard over that section of the Wall (while the Watch's headquarters were transferred to the more centrally located Castle Black). Deep Lake castle was paid for by Alysanne herself, and constructed by men that Jaehaerys I sent north. Thus the Night's Watch also remembers Jaehaerys and Alysanne with great esteem, as increasingly later monarchs came to practically ignore the Night's Watch.

Jaehaerys I ruled wisely and well until the end of his long reign. Towards the end of his life he was confined to bed and apparently suffered some form of Dementia as he would regularly mistake Alicent Hightower (the young daughter of his Hand Ser Otto Hightower) as one of his daughters. Jaehaerys died of old age while Alicent was reading a book to him. Jaehaerys lived so long that he actually outlived Alysanne by several years as well as two of his own sons (who may have died young), and was thus succeeded by his grandson Viserys I - the only time in history that a king was succeeded by his grandson.