Aegon IV Targaryen

Aegon IV Targaryen, also called Aegon the Unworthy, 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.

Aegon IV was the fourth king of his name to sit on the Iron Throne and the eleventh king of the Targaryen dynasty to rule the Seven Kingdoms. He was an ancestor of Viserys and Daenerys Targaryen. He was formally styled Aegon of House Targaryen, the Fourth of His Name, King of the Andals and the First Men, Lord of the Seven Kingdoms and Protector of the Realm.

Background
Aegon IV fathered several children, both trueborn and bastard. He was the father of Daeron and Daenerys by his sister-wife, Queen Naerys. With his cousin and mistress, Queen Daena, he fathered Daemon Blackfyre. He succeeded his father Viserys II Targaryen and was, in time, succeeded by his son Daeron II.

Season 1
Aegon IV, along with his vast progenie, is mentioned in the House Targaryen's entry of the book The Lineages and Histories of the Great Houses of the Seven Kingdoms

In the books
In the A Song of Ice and Fire novels, Aegon IV is remembered as one of the worst Targaryen kings, which is the reason why he is popularly known as "Aegon the Unworthy". He was a decadent ruler and glutton who indulged his passions and whims at every opportunity. Being attractive in his youth, he was popular with women both highborn and lowborn. He had as many as nine mistresses and many bastards. Supposedly, he had any woman he wanted whether they were married or not. People often joked that while the Targaryen words were "Fire and Blood", Aegon IV's motto might as well have been "Wash her and bring her to my bed". Due to his years of overindulgence, by the end of his reign Aegon IV was extremely obese, to the point that his legs could barely support his own weight. Four of his bastard children were born to noblewomen, and they became known as the "Great Bastards". Chief among them was Daemon, who upon being legitimized created House Blackfyre. Other notable bastards were Aegor Rivers ("Bittersteel"), Brynden Rivers ("Bloodraven") and Shiera Seastar. Aegon IV foolishly legitimized all of his bastard children upon his deathbed, which resulted in Daemon Blackfyre declaring himself the rightful heir and tearing apart the Seven Kingdoms in civil war. Ultimately, Aegon IV's corruption led to a series of Blackfyre Rebellions which would plague Westeros for five generations.

According to author George R.R. Martin, Aegon IV is essentially the Targaryen equivalent of Henry VIII. Martin's instructions for artistic depictions of Aegon IV explicitly cite Henry VIII. Both kings started out young and attractive, but by the ends of their reigns became infamously fat from overindulgence, and their numerous sexual affairs resulted in bloody succession conflicts which were eventually resolved with the rise of good rulers (Daeron II and Elizabeth I respectively).