Brynden Rivers

Brynden Rivers 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.

Background
Brynden was a bastard son of King Aegon IV Targaryen, also known as "Aegon the Unworthy".

Season 1
Brynden, along with his siblings is mentioned in the House Targaryen 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, Brynden Rivers was often called "Bloodraven". While King Aegon IV had many bastard children, the mothers of four of them were also noblewomen, which encouraged him to formally acknowledge them. The four Great Bastards were Daemon Blackfyre (whose mother, secretly, was Daena Targaryen), Aegor "Bittersteel" Rivers (whose mother was a member of House Bracken), Brynden "Bloodraven" Rivers (whose mother was a member of House Blackwood, and Shiera Seastar (whose mother was Serenei of Lys, from an ancient Valyrian noble family).

On his deathbed, Aegon IV foolishly had them all legitimized, which led to strife between them and his legitimate heir King Daeron II, tensions which ultimately split realm apart in the devastating civil war known as the Blackfyre Rebellion. Bloodraven sided with their half-brother King Daeron II Targaryen, while his other half-brother, Bittersteel sided with his half-brother Daemon Blackfyre.

Bittersteel and Bloodraven hated each other, because they both competed for the love of their half-sister Shiera Seastar. Bloodraven asked her to marry him dozens of times and she refused, but she did let him into her bed - though she had many lovers. Shiera, however, completely rejected Bittersteel, and for this he never forgave Bloodraven. It also didn't help that Bittersteel's mother was a Bracken and Bloodraven's mother was a Blackwood, as those two noble families had been feuding for thousands of years. Bittersteel became a great champion and general for the Blackfyres, while Bloodraven became a great champion and general for the Targaryens.

The First Blackfyre Rebellion ended in 196 AL with the Battle of Redgrass Field, in which Daemon Blackfyre and his eldest two sons were slain. Bittersteel survived, and became one of the major leaders of the surviving Blackfyre forces as they fled into exile in the Free Cities. Daemon left behind five sons and multiple daughters, so Bittersteel became their guardian, secreting them across the Narrow Sea with his dwindling followers. They would continue to harass the Seven Kingdoms with plots and failed rebellions for decades. Brynden lost an eye fighting Bittersteel at Redgrass field, but later in the battle, it was Bloodraven's private company of archers, known as the Raven's Teeth, who rained arrows down upon Daemon Blackfyre and his two eldest sons, killing them. Many called Bloodraven a kinslayer for this (though no one knew exactly whose arrows had killed Daemon - he was pierced by many). After King Daeron II died in the Great Spring Sickness of 209 AL he was succeeded by his son Aerys I Targaryen - who proved to be a weak, absentee-king, more concerned with introverted scholarly pursuits than any form of governing. When Aerys I was crowned, however, Bloodraven was named as his new Hand of the King, a position he held throughout Aerys I's twelve-year reign. During that time, Bloodraven was the real power behind the throne, and the functional ruler of the Seven Kingdoms. Aerys I's rule was deeply troubled by plague, drought, and insurrection, and many blamed Bloodraven because he was in charge, but ultimately he was the man holding the realm back from the brink of chaos during this troubled period.

During his time as Hand of the King, Brynden was not only an effective administrator and strategist, but a cunning spy-master, and the information from his secret agents greatly aided his grip on power. His spies were so widespread that, due in part to the eye he lost in battle, a popular riddle at the time asked, "How many eyes does Bloodraven have?" to which the answer was "A thousand eyes...and one..."

Aerys I died in 221 AL without issue, so he was succeeded by his younger brother Maekar Targaryen. For undisclosed reasons, Maekar distrusted Bloodraven, so when he was crowned he had him stripped of his position and confined to the Black Cells under the Red Keep for an unknown amount of time. Maekar ruled for twelve years and was then succeeded by his own son, Aegon V Targaryen, also known as "Egg". In 233 AL, the year he was crowned, Aegon V had Bloodraven sent north as part of the "honor guard" that escorted his older brother, Maester Aemon, to the Wall. Bloodraven remained there and along with Aemon, joined the Night's Watch. In time, Bloodraven rose to become the new Lord Commander. He served long and well in this position, but one day he went out along ranging beyond the Wall, and mysteriously never returned.