Maester Aemon, formerly known as Aemon Targaryen, was a Maester and Sworn Brother of the Night's Watch. He served as the Maester for Castle Black under Jeor Mormont and Jon Snow. He died of old age on a journey to Oldtown.
Aemon was the third son of King Maekar I Targaryen, who was himself a fourth son. Aemon was named after his great-great uncle, Prince Aemon Targaryen by his grandfather, King Daeron II Targaryen. Aemon forged his chain, and earned the title of Maester Aemon.
He served an unidentified lord until his father summoned him to court to serve the Iron Throne. Not wanting to displace the current Grand Maester, Aemon chose instead to serve at the keep of his eldest brother, Prince Daeron.
After Daeron's death, Maester Aemon's whereabouts are unknown until the death of his father, when Aemon was summoned to court in the year of the Great Council. He was offered the crown quietly, but refused, ceding rule to his younger brother, Aegon. He then decided to serve as a black brother of the Night's Watch for fear that he may be used in a plot to usurp his brother.
A Game of Thrones
After attacking Jon Snow in the armoury, Grenn and Todder are sent to maester Aemon by Donal Noye, to check upon their injuries. Aemon later attends a meeting of the officers at Castle Black, for Tyrion Lannister's last night at the Wall. There, he and Tyrion have a brief exchange, where he describes Tyrion as a giant, and is in response called kind by the dwarf.
A Feast for Crows
Jon sends Aemon away from Castle Black with Sam, Gilly and Gilly's son. He wants to purge the castle of people that Melisandre might use in her blood magic, as she requires king's blood, something that Aemon possesses.