Aegon III Targaryen

"Rhaenyra Targaryen was murdered by her brother, or rather, his dragon. It ate her while her son watched. What's left of her is buried in the crypts right down there."

- King Joffrey Baratheon points out what little remains of Rhaenyra in the Great Sept of Baelor

Aegon III Targaryen is an unseen character in Game of Thrones. He is long dead by the time of the series. He was a King of the Targaryen dynasty.

Background
Aegon III was a King of the Targaryen dynasty.

Season 3
While touring the Great Sept of Baelor in King's Landing, King Joffrey Baratheon explains to Margaery Tyrell that Rhaenyra Targaryen was eaten by her brother's dragon while her son - Aegon III, whom he doesn't mention by name - was force to watch.

In the books
In the A Song of Ice and Fire novels, Aegon III watched as his mother was devoured by the dragon of Aegon II, his uncle, during the civil war known as the Dance of Dragons. Due to the trauma of witnessing his mother's death, Aegon III developed a fierce hatred and terror of the creatures.

Aegon III ascended to the Iron Throne after Aegon II died without any issue. He ruled the Seven Kingdoms long and well, though he is often blamed for the death of the last dragon, for which he became known as "Aegon the Dragonbane" despite his later efforts to obtain Dragon eggs. At some point he had nine mages cross the sea to attempt to hatch his remaining cache of eggs.

Aegon had two sons Daeron, called the Young Dragon, and Baelor, called Baelor the Blessed, and among his daughters were the three "Maidens in the Tower," Daena the Defiant, Rhaena and Elaena. Upon his death in 157AL, Aegon III was succeeded by his young son Daeron. Aegon was said to have always walked with a great grief, and the folly of his sons nearly tore the realm apart.