XoXaan's holocron was created by the Sith Lord XoXaan. XoXaan was one of the Dark Jedi exiled from the Republic after their defeat in the Hundred-Year Darkness. Her holocron was lost in the Tombs of Korriban.
Millennia after her death, former Jedi Master A'Sharad Hett accidentally discovered the Holocron and, pretending to be a Sith Acolyte, learned of the dark side of the Force from the Holocron's gatekeeper, and eventually became the student of her teachings in truth, forming the One Sith.