Aegon V Targaryen

Aegon V Targaryen, also called Aegon the Unlikely and informally as Egg, is an unseen character in Game of Thrones. He is long dead by the time the series begins.

Aegon V was the fifth king of his name to sit on the Iron Throne and the fifthteenth king of the Targaryen dynasty to rule the Seven Kingdoms. He was the father of the Mad King, the grandfather of Viserys and Daenerys Targaryen and the younger brother of Maester Aemon Targaryen. He was dubbed 'the Unlikely' because he was the fourth son of a fourth son and thus unlikely to have inherited the crown.

Season 1
At Castle Black, Maester Aemon tells Jon Snow about how the children and grandchildren of his brother 'Egg' were slaughtered during Robert Baratheon's rebellion. Snow realises that Aemon is a member of House Targaryen.

In the books
Aegon V Targaryen is a major character in George R.R. Martin's 'Tales of Dunk and Egg' prequel novellas, which are set around ninety years before the events of A Song of Ice and Fire.

Aegon, using the alias 'Egg', spent many years travelling the Seven Kingdoms as a squire in the service of Ser Duncan the Tall, a hedge knight. At their first meeting Duncan was unaware of Egg's true identity, until he used his royal authority to save Duncan from being killed by his bullying older brother, Prince Aerion. Duncan was able to instead face Aerion on the tournament field and defeat him in combat. Unfortunately, Prince Baelor 'Breakspear' Targaryen, the royal heir, was killed during the tournament. Prince Maekar - Aegon's father - despaired of Aerion's spoiled indolence and agreed to let Aegon serve as Duncan's squire to build character and gain experience of living in the 'real' world.

Aegon ascended to the Iron Throne in 233 AL, following his father Maekar's death in battle with an outlaw lord. Of Aegon's three older brothers, Aerion had poisoned himself to death (by ill-advisedly drinking wildfire, thinking it might turn him into a dragon), Daeron had died of a pox and Aemon had disinherited himself by first joining the maesters and later the Night's Watch. As Aegon V Targaryen, he ruled for twenty-six years. During this time he made Ser Duncan the Lord Commander of the Kingsguard and secured an alliance between the Targaryens and House Baratheon by wedding his daughter Rhaelle to the then-Lord of Storm's End (she became the grandmother of Robert, Stannis and Renly Baratheon, giving the Baratheons their claim to the Iron Throne). He had two sons, Duncan (often called Duncan the Small) and Jaehaerys.

Aegon V ruled wisely and well, but he had two major weaknesses. The first was an indulgence with his sons. Having married for love himself, he permitted his sons to do the same, although this was not politically wise and made him enemies within the Seven Kingdoms. He also had an overwhelming obsession with hatching the surviving Targaryen dragon eggs. Both weaknesses culminated in 259 AL at the so-called 'Tragedy at Summerhall'. This was a fire that got out of control and destroyed the castle, killing Aegon V, Ser Duncan the Tall and Prince Duncan the Small. On the same night, Aegon's great-grandson Rhaegar Targaryen was born.

Behind the scenes
For the TV series, the Targaryen lineage was simplified. Aegon V's younger son Jaehaerys, who inherited the throne as Jaehaerys II and ruled for three years before dying of grief, was eliminated from the line of succession and the Mad King, Aerys II, was instead made Aegon V's younger son directly. This makes Aegon V the grandfather of Viserys and Daenerys Targaryen, and Maester Aemon their grand-uncle.

So far, it has not been confirmed in the series if Aegon V's daughter did marry into House Baratheon, giving the Baratheons their claim to the throne in the TV series as in the books.