There were plenty of theories going into Rebecca Yarros' Onyx Storm — many of which considered what Violet's second signet could be, or what would happen after Xaden turned venin — but one of the most prominent and unanimously agreed upon was the idea that Naolin, Tairn's previous rider, was the Venin Sage. Naolin's character is one that has been continuously referred to, but given very little backstory or attention thus far, so it's no wonder theories suggested he'd pop up later in the Empyrean Series' novels.