HBO is expanding the Game of Thrones universe with a story about two niche characters that have a basis in George R. R. Martin's novellas. The key characters, i.e., the Hedge Knight, Ser Duncan the Tall, and Aegon/ Egg are mentioned in A Song of Ice and Fire series. They appear in The Hedge Knight - the prequel novella to Martin's ASOIAF, and the source material of HBO's latest.