First Timeless Child

The "Timeless Child" was a truth hidden by the Time Lords. A secret too devastating to learn, the discovery caused to ravage Gallifrey, apparently killing off the Time Lords. (TV: Spyfall)

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)