Aerion Targaryen

"Over there in that urn, the ashes of Aerion Targaryen. Aerion Brightflame they called him. He thought drinking wildfire would turn him into a dragon:  he was wrong."

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

Aerion Targaryen is an unseen character in Game of Thrones. He is long dead by the time of the series and only appears in Histories & Lore. He was a Prince of the Targaryen dynasty.

Background
Aerion was a Prince of the Targaryen dynasty. His father was King Maekar I Targaryen. He was the older brother of Aemon and Aegon Targaryen, who would ascend to Iron Throne as Aegon V.

Aerion killed himself by drinking wildfire, believing it would transmute him into a dragon.

Season 3
Touring the Great Sept of Baelor in King's Landing, King Joffrey Baratheon points out to Margaery Tyrell the urn containing what little remains of Aerion Brightflame. He briefly explains who he was and the manner of his death.

In the books
In the A Song of Ice and Fire novels, Aerion Targaryen is remembered as an example of the madness that can befell the members of House Targaryen, believing himself a dragon in human form, and had a rather cruel streak that would lead him to be nicknamed Aerion the Monstrous. In his youth, Aerion was slim and of average height with curly silver-hair, a statuesque face of sharp cheekbones, straight nose, pale skin and violet eyes.

Aerion's brothers were not fond of him, particularly Aegon. At some point of Aegon's childhood, Aerion threw his cat down a well and stole into his chambers with a knife and threatened to castrate Aegon to get a sister he could marry. After the events of the Ashford Tourney, which led to the death of Maekar's brother Baelor, Aerion was sent to Lys, where he may have fathered some children. During his exile he also served with the Second Sons, a mercenary company.

After the death of Maekar, Aerion's son was passed over by the Great Council that would choose his successor as it was feared the son may have inherited his father's madness.

Aerion Brightflame is one of the main antagonists in the first entry in the "Tales of Dunk and Egg" series of prequel novellas set 90 years before the War of the Five Kings. He is a major opponent of Ser Duncan the Tall, and challenges him to a trial by combat - or rather, a rare "trial of seven" variant in which two sides of seven men fight each other. Aerion Brightflame would thus appear in any potential TV adaptation of the prequels.