Fourth Timeless Child

According to the Spy Master, the Founding Fathers of Gallifrey built a lie about the existence of the Time Lords on the Timeless Child. It was buried deep in the memories and identity of all Time Lords. (TV: The Ghost Monument, Spyfall)

Upon learning the truth concerning the Timeless Child, the Master ravaged Gallifrey and claimed to have wiped out the Time Lords to "make them pay". He told the Thirteenth Doctor that learning the truth had not been easy for him. (TV: Spyfall)

The Remnants referred to the Timeless Child when drawing on the Thirteenth Doctor's fears and history on the planet Desolation. When they made the reference, the Doctor was confused for a moment before she angrily told the Remnants to "get out of [her] head". (TV: The Ghost Monument)

When the Master first mentioned the Timeless Child mystery to the Doctor, she got a massive headache and a flashback to when the Remnants approached her as well as a glimpse of a child on a purple-skied planet. (TV: Spyfall)