The "Timeless Child" was a truth hidden by the Time Lords. A secret too devastating to learn, the discovery caused the Spy Master to ravage Gallifrey, apparently killing off the Time Lords. (TV: Spyfall)
The Eleventh Doctor had previously hinted about knowing this, having once stated he knew "secrets; knowledge that must never be spoken." (TV: The Rings of Akhaten) The Twelfth Doctor said a secret was told to him by the Cloister Wraiths that never left him right in the head again. (TV: Hell Bent) The Remnants referred to the Timeless Child when drawing on the Thirteenth Doctor's fears and history, on the planet Desolation. When the Remnants made the reference, the Doctor was stunned for a moment before she angrily told the Remnants to get out of her mind when they taunted her. (TV: The Ghost Monument)