Saxon Master's regeneration

The Saxon Master was forced to regenerate after being stabbed by Missy, his future incarnation. (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
On her Spacebook profile, Missy acknowledged that she had died eighteen times before her current incarnation, not including stealing bodies. (PROSE: Girl Power!)