Sissiri, a Sith Lord of old, had their image sculpted into the stone surfaces of the Exegol citadel around 35 ABY. Kylo Ren, who was both the Supreme Leader of the First Order and the Master of the Knights of Ren, journeyed to Exegol to face Darth Sidious; while there, he passed through the Sith Citadel and identified several statues of Sith Lords that had been etched into the walls, figures he knew from his research.
Among the Sith Lords he recognized were Locphet, Mindran, Felkor, Naga Sadow, and Sissiri. During his time there, Kylo Ren also perceived their incomprehensible murmurs, along with Sidious' well-known voice, which he had been hearing since his youth.
The first mention of Sissiri occurred within the pages of Star Wars: The Rise of Skywalker: A Junior Novel, a 2020 book written by Michael Kogge.