After leaving the FBI for a second time, Black became embroiled in a custody battle (with his wife's family) for Jordan. He had himself admitted to a psychiatric institution for assessment, to counter their claims that he had a "millennial obsession" which would make him an unfit guardian. Around Christmas, 1999, he was approached by two FBI agents, Dana Scully and Fox Mulder, who wanted his help on a case involving the Millennium Group and the seeming reanimation of group members who had committed suicide. Initially refusing to get involved, he lent them cryptic clues and eventually helped save Agent Mulder, who was trapped in a basement with the walking dead.
(source: The X-Files, episode: "Millennium")