The Lumiat's regeneration

The Lumiat, a benevolent incarnation of the Master, was forced to regenerate after being shot by her previous incarnation, Missy.

History
Following her previous incarnation's regeneration via an Elysian field, the Lumiat became the embodiment of Missy's last act, being in the name of good and kindness.

She chose an attempt at influencing Missy at an earlier point in her timeline to change her evil ways, which resulted in Missy growing tired of the Lumiat and eventually shooting her and leaving her on a random planet to regenerate. (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, The Doctor vs the Master,  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.