Saxon Master's regeneration

The Saxon Master was forced to regenerate after being stabbed by Missy, his future incarnation.

A day to come
Following the Year That Never Was, the Saxon Master was shot by Lucy Saxon but refused to regenerate, choosing to die rather than live with the Tenth Doctor, who proceeded to burn the Master's body. (TV: Last of the Time Lords) However, the Master had made arrangements for the Disciples of Saxon to resurrect him using salvaged biometrical signatures. This process was interrupted by Lucy Saxon, resulting in the resurrected Saxon Master burning up his own life force. He was embroiled in the Ultimate Sanction, which ended with him returning to Gallifrey at the end of the Time War as he launched an attack against Rassilon. (TV: The End of Time)

The Moment foresaw that the battle between the Master and Rassilon would end in both regenerating, with the "crewcut" Rassilon regenerating into a bald incarnation. (PROSE: Pandoric's Box) However, the Twelfth Doctor came to understand that the Master was kicked off Gallifrey by the Time Lords after they had cured his "little condition". (TV: The Doctor Falls)

The Regeneration
Following his request to follow him back to his TARDIS via the lift system of the colony ship was denied by Missy, she asked him to come up close to her. As they hugged, she reminisced about how she "loved being [him]. The way [he] burned like a Sun, like a whole screaming world on fire." before she stabbed him in the back with her dagger.

Helping him back to lift door, she told him she "was precise", giving him time to reach his TARDIS and "maybe even get a cuppa". Asking her why she did it, Missy announced that it was time "to stand with the Doctor" and that it was always where they were going.

As she started to walk away, the Master began screaming that he would "never stand with the Doctor" as he shot her with his laser screwdriver, and told her that she should not bother trying to regenerate as she got the "full blast", and that "this is our perfect ending, we shoot ourselves in the back." And as he left her to die on the ground, he went into the lift, heading downwards towards his TARDIS. (TV: The Doctor Falls)

Attempt at prevention
In an aborted timeline, the Master attempted to avoid his regeneration into Missy by healing himself, first with the energies of Kiameth, releasing a sentient entropy wave on the universe in the process, and then by feeding on his own past incarnations who he time-scooped to the ruins of Kiameth, coming into conflict with Missy. These events were undone when the "War" Master created a paradox by poisoning all their past selves including himself and letting the wave consume Missy, having deduced the wave was actually the Master's own future form. (AUDIO: Masterful)

Legacy
Missy referred to her change from male to female as an "upgrade", (TV: The Witch's Familiar) a term also used by River Song, who questioned Missy about it. Missy claimed to have had no control over the regeneration, that she "just woke up one day, even more fascinating than usual". (AUDIO: The Bekdel Test)

On her Spacebook profile, Missy acknowledged that she had died eighteen times before her current incarnation, not including stealing bodies. (PROSE: Girl Power!)