While Millennium was cut short after three seasons, that wouldn't be the last of Frank Black, as he would go on to crossover with The X-Files. Over the course of his career, TV writer/producer Chris Carter has been lucky enough to see several of his ideas blossom into full-fledged series, most notably The X-Files, which ran for nine seasons in its original run. Unfortunately, most of Carter's other shows flamed out pretty quickly, some not even making it a full season. One project that managed to find a following though was Millennium.
Premiering in 1996, Millennium ended up a bit of an odd duck, running for three seasons, but with all three of them being noticeably different. Season 1 of Millennium was essentially an extremely dark police procedural with supernatural elements, as Frank hunted down brutal serial killers for the FBI-affiliated Millennium Group. Season 2 got much more overtly supernatural and metaphysical, with the introduction of literal demons, and ending in a manner somewhat akin to a biblical apocalypse. Despite that apparent ending, season 3 happened, ending up much more in line with season 1 tonally, but with Frank getting a new partner and now being an enemy of the Group.
While season 3 of Millennium ended on a bit of a cliffhanger, thankfully, that wouldn't be the last time fans got to see Frank, as he would go on to appear in a crossover episode of The X-Files, and later join up again with Fox Mulder in an officially sanctioned comic book series.
How Millennium Crossed Over With The X-Files (After It Ended)
Millennium's series finale - which again, wasn't written to be one - saw Frank Black and his daughter Jordan go into hiding out of fear of the Millennium Group. Eight months later though, in The X-Files' season 7 episode "Millennium," Frank has checked himself into a mental hospital, and sends Jordan to live with her maternal grandparents. The Millennium Group is thought to have dissolved, but Frank isn't so sure, and soon, Mulder and Scully approach him for to help with a case possibly connected to the remnants of the organization. Frank is reluctant to get involved, but eventually does, and what's left of the Group is seemingly defeated. Jordan reunites with her father, and they celebrate the turning of the millennium, while Mulder and Scully share a kiss.
That wasn't the end of Millennium and X-Files crossovers though. In 2015, a five-issue Millennium comic book miniseries was released, which took place a decade after Frank Black's appearance on The X-Files. Frank comes to discover that the Millennium Group has returned, and Jordan - she and her father had become estranged - is now a willing member. Jordan uses her now fully manifested psychic abilities to combat Frank's old demonic enemy Lucy Butler, while Mulder joins Frank's crusade against the Group, ending up almost killed by Lucy in the process. Jordan seems to be trying to set the Group back on the right path, taking out Lucy, then leaving with them. Mulder tries to talk Frank into coming back to the FBI, but Frank rebuffs him. Oddly, Scully doesn't play a role in the series.