After lifting the timey-wimey lid on the Timeless Child, Doctor Who has a brand new secret with The Division. The season 12 finale of Doctor Who represented arguably the most significant addition to franchise canon for decades, as it ran headfirst into The Doctor's personal backstory and gleefully played havoc with her timeline. "The Timeless Children" revealed that The Doctor originally came from another universe, travelling here from an unknown place via unknown means. Rescued by a scientist from Gallifrey, The Doctor's unique biology was stolen to create the Time Lord regeneration process and The Doctor herself went through a whole host of different incarnations that she currently doesn't remember.
This is revealed to be the work of The Division. Although the files within the Time Lord Matrix are frustratingly incomplete, The Doctor learns that, at some point during her youth, she scored incredible test results and was invited to work for a clandestine Time Lord offshoot agency called The Division, an organization that goes against Gallifrey's usual policy of non-interference to directly encroach upon the natural flow of history. After serving The Division, The Doctor's memories are wiped and, presumably, this is where The Doctor's life, as the audience knows it, begins.
Clearly, The Division are going to play a massive part in the future of Doctor Who and are likely the next big mystery now that the Timeless Child arc is over, but who are they? Although their name is only revealed in the finale, The Division have appeared before in season 12. The first occasion came in "Fugitive of the Judoon," when Jodie Whittaker's Doctor meets Commander Gat, the woman hunting down the Ruth Doctor. It appears that Ruth and her partner tried to escape working for The Division, but were ultimately tracked down. This episode proves The Division have a militaristic structure, with medals and ranks, and are happy to use lethal force. Gat also alludes to a higher power at the top of The Division's chain of command.
The second time viewers see The Division is via the "Brendan" scenes in "Ascension of the Cybermen." As revealed by The Master, the image of a boy growing up in Ireland was used as a visual filter to overlay The Doctor's true origin story hiding deep in the Matrix. The Doctor/Brendan was found and adopted as a baby, suffered a deadly fall but survived, was recruited into the Garda/Division, and then was mind-wiped in the back room of a police station/TARDIS. These scenes reveal that The Doctor's adoptive mother, Tecteun, was complicit in The Division recruiting The Doctor and, most likely, The Doctor was accepting of the memory-wiping process, despite the pain it causes.
The Division isn't the first secretive Time Lord interference agency in Doctor Who, however. Introduced in Fourth Doctor adventure "The Deadly Assassin," the Celestial Intervention Agency were a group hidden from even Gallifrey's High Council, who would interfere in history as they saw fit to protect the desired end outcome. Although much of the CIA's role in Doctor Who takes place in audio and written adventures, the agency are generally accepted to be the ones responsible for sending The Doctor on Time Lord missions, such as destroying the Daleks in "Genesis of the Daleks." It's currently unclear whether The Division is Chris Chibnall's retcon of the CIA, or whether both are the same group, using different names at different points in history. Certainly, it would be difficult for the CIA and The Division to both exist in Doctor Who's world.
Although presented mysteriously, the overall intentions and tone of The Division can be more or less entirely gleaned from Doctor Who season 12. They're the formidable, unaccountable, time-altering secret service of Gallifrey. What needs answering is The Doctor's relationship with the group, and what they made the original Time Lord do while under their employ? Now that The Doctor knows of The Division's existence, she'll no doubt be on a mission to uncover those exact details, and it wouldn't be a surprise to see The Division become the overarching villains of Jodie Whittaker's next season.
Doctor Who returns with "Revolution Of The Daleks" this holiday season on BBC and BBC Ameirca.