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