Aegon II 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 II 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 II was a King of the Targaryen dynasty.

Season 3
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 was force to watch. Joffrey points out the crypt where what little is left of her is kept.

In the books
In the A Song of Ice and Fire novels, Aegon II took the Iron Throne at the behest of Lord Commander Criston Cole of the Kingsguard, thereafter known as Criston the Kingmaker, even though his father, Viserys I, intended Rhaenyra to succeed him. Rhaenyra disputed his ascension and claimed the throne for herself, leading to the civil war known as the Dance of the Dragons. Aegon managed to defeat his sister, and fed her to his dragon. Shortly thereafter he died himself, leaving no male heirs, which led to his nephew Aegon III, Rhaenyra's son, taking the throne.