Elysian field

An Elysian field was a method, using cryogenetic extraction, that allowed a Time Lord to completely break down their body into its components, i.e. atoms and molecules, and rebuild themselves, editing their personality. It could spark regeneration, even at the end of a regeneration cycle, and thus was banned by the Time Lords. (AUDIO: The Lumiat)

When her shot her, wounding her beyond the ability to regenerate, (TV: The Doctor Falls)  used an Elysian field to survive and regenerate, removing all her "snide" evil and leaving only the kindness, in a final act of breaking the rules, in the hopes of impressing the Doctor. This new incarnation was almost entirely good, but did not retain very many of her memories. Leaving the name the Master behind, she choose to call herself. (AUDIO: The Lumiat)

Behind the scenes
In Greek mythology, the Elysian Fields were the section of the afterlife reserved for the heroic and pure of heart.