Of those he recognized were Locphet, Mindran, Sissiri, Naga Sadow, and Felkor. While there, Kylo Ren also heard their unintelligible whispers, along with the familiar voice of Sidious, which he had heard since childhood.
Felkor was first mentioned in Star Wars in the 2020 novel Star Wars: The Rise of Skywalker: A Junior Novel, authored by Michael Kogge.