The World of Ice and Fire (US, UK) will be released October 28. Editors, please refrain from including information from the book prior to November 27.
From A Wiki of Ice and Fire
Ch'Vyalthan was an archmaester of the Citadel who was credited with the authorship of An History of the Great Sieges of Westeros. He is deceased by the time of King Robert Baratheon's death.
He is mentioned in the second season of the Game of Thrones television series in the episode "The Prince of Winterfell".