Timeless Child

The Timeless Child was a title used to refer to the mysterious being from whom, across several regenerations, the First Tecteun extracted the secret of regeneration. According to the Spy Master, the Timeless Child's incarnations were none other than the original incarnations of the Doctor, long before the First Doctor.

Discovery
The Timeless Child was a child discovered by Tecteun on another planet, who had the power to regenerate, and had been waiting beneath a wormhole (TV: The Timeless Children) to Universe Two. This universe was theorised by Tecteun to have been the Timeless Child's realm, although the Thirteenth Doctor was less certain that this was the case. (TV: Survivors of the Flux)

The child was adopted by the First Tecteun, who discovered her power to regenerate after she suffered a fatal fall from a cliff, leading to Tecteun studying the child's power for a number of years and regenerations. (TV: The Timeless Children) During this time, the child referred to Tecteun as "Mother", while she regarded them as her "experiment". (TV: Survivors of the Flux)

Tecteun was eventually able to replicate this regenerative ability, regenerating herself into a new incarnation. The Second Tecteun eventually spread this power to other Shobogans, who would go on to become Time Lords, and artificially limited the number of regenerations to a maximum of twelve.

In the process, the Timeless Child became the genetic template upon which all Time Lords were modeled, giving them their characteristic regenerative abilities. (TV: The Timeless Children)

Service to the Division
Tecteun and the child were inducted into a clandestine organisation called the Division, led by a Time Lord called Solpado, whose job it was to intervene in time when necessary. After being tested, the results came back saying they were the best the Divison have ever seen. (TV: The Timeless Children)

Legacy
Richard Francis Burton once guessed that the Great Houses (PROSE: The Book of the War) of the Time Lords (PROSE: Lungbarrow, et. al) had achieved the power of regeneration by modifying themselves with the "taint" of the Yssgaroth, (PROSE: The Book of the War) much like how the account involving Tecteun and the Timeless Child involved the former taking biodata from the child and applying it to Time Lord DNA. (TV: The Timeless Children) Indeed, the vampires active in the universe were searching for a Child-That-Was-Taken. (PROSE: Out of the Box)

According to the Spy Master, the Founding Fathers of Gallifrey built Time Lord society on a lie regarding the Timeless Child. This was buried deep within all Time Lords, in the form of repressed thoughts and memories: hidden, but integral to their identities. (TV: Spyfall) He later expanded upon this stating that an individual known as the "Timeless Child" was discovered and whose ability to regenerate was written into the genetic code of Time Lords. This history was subsequently hidden, with another explanation being given for this ability instead. This was the mythology all Time Lords believed was their history. (TV: The Timeless Children)

To contain the secret, large portions of the Matrix on Gallifrey pertaining to the life of the Timeless Child were redacted, beyond the Master's ability to recover them, even after the fall of Gallifrey. However, Tecteun hid some of the truth in the Matrix in the form of the story of Brendan, an immortal Irish policeman. The Master suggested that this was an attempt by Tecteun to apologise, or possibly meant as a gift, to help her child one day work out the truth of their existence. (TV: The Timeless Children) Tecteun herself told the Thirteenth Doctor she did not wish them to find out, however, as she feared the Doctor's reaction. (TV: Survivors of the Flux)

While taunting the Thirteenth Doctor about her fears, Remnants on the planet Desolation looked "further back" into her past and referred to the Timeless Child. When they made the reference, the Doctor was confused and asked them what they were talking about, then angrily told the Remnants to "get out of [her] head". (TV: The Ghost Monument)

Once the Master learned the truth about the Timeless Child, he ravaged Gallifrey to "make them pay" for what he discovered. He told the Thirteenth Doctor that learning the truth had not been easy for him. (TV: Spyfall) He was enraged that every Time Lord had a piece of the Doctor in them, including him.

On Gallifrey, he told the Thirteenth Doctor the truth about her past. (TV: The Timeless Children) She suffered an identity crisis, recovering on her next battle with the Daleks, (TV: Revolution of the Daleks) who had been central to the formation of the First Doctor's identity. (TV: Into the Dalek)

She pursued the last known member of the Division, Karvanista, (TV: The Halloween Apocalypse) and at the onset of the Flux, relived the Siege of Atropos from the Fugitive Doctor's perspective from within her time stream. (TV: Once, Upon Time) When the Doctor was recalled by the Weeping Angels, a later incarnation of Tecteun confirmed the story of how she found her, and explained that the Flux was engineered to destroy the Doctor's universe specifically in response to the Doctor's influence.

She regarded the Doctor, having escaped the Division, as a virus, always coming in the way of the Division's plans in the course of their travels. The universe was fundamentally changed by their influence. As a result, Tecteun looked to destroy this universe, and cross over into another. (TV: Survivors of the Flux)

Behind the scenes
Author Aristide Twain wrote the mention of a "Child-That-Was-Taken" into Out of the Box in reference to the fan theory that the Timeless Child may have originated from the Spiral Yssgaroth, theorising that the First Tecteun's abduction of her could be the secret reason behind the conflict between the Great Vampires and Gallifrey. Nonetheless, the story lacked the legal ability to directly reference the Timeless Child, leaving it possible the "Child-That-Was-Taken" references a different child. Indeed, Twain has gone on record to say "if you’d rather believe the Child-That-Was-Taken is something else entirely, who am I to stop you?".