Missy's regeneration

Aboard a Mondasian colony ship, Missy was mortally wounded by the "full blast" of the laser screwdriver wielded by her previous incarnation, the Saxon Master, forcing her to create an Elysian field to regenerate into the Lumiat.

History
The Saxon Master, who was mortally wounded by Missy after refusing to join the Twelfth Doctor in the Battle of Floor 0507, intended to prevent Missy from regenerating, leaving her to die as he took his leave towards his TARDIS, himself regenerating. (TV: The Doctor Falls)

Missy, however, managed to regenerate by creating an Elysian field to "kick start a new regeneration cycle". The process of cryogenetic extraction destroyed her body but retained a copy of her consciousness before regeneration, creating a new incarnation, who became known as the Lumiat, though many memories specific to Missy were lost. (AUDIO: The Lumiat)