The Lumiat's regeneration

The Lumiat was forced to regenerate after being shot by her previous incarnation, Missy. (AUDIO: The Lumiat)

The Thirteenth Doctor understood that the incarnation who introduced himself as the Spy Master was in the future relative to Missy. (PROSE: The Wonderful Doctor of Oz, TV: Spyfall)

Behind the scenes

 * The ending of The Lumiat seemingly implied that the Lumiat would regenerate into the Spy Master, who made his debut on television shortly before the release of the story. This was a coincidence, as the script was written and recorded long before the plans for Sacha Dhawan to take over.