- "Shame Archmaester Ch'Vyalthan wasn't a better writer."
Ch'Vyalthan, is an unseen character mentioned in the second season. He is deceased when the events of the series begin and is not expected to appear in the series. He was an Archmaester of the Citadel and the author of the book An History of the Great Sieges of Westeros.
Tyrion Lannister refers to his book when preparing the defence of King's Landing. Varys criticizes him as a boring writer. Tyrion and Bronn debate how to correctly pronounce his name, but when Varys arrives he reveals that it is pronounced exactly as it is spelled: "Chuh-vye-all-than". 
In the Books
No mention of an Archmaester Ch'Vyalthan is made in the A Song of Ice and Fire novels.