Child Master's degeneration

"Ahhhhhhhhhh!!!! You did this, Doctor! You and your blasted companion! I curse you alllllll!!!! Revengggggggeeee!!!!"

- Child Master's last words

Having regenerated into a child incarnation during the Last Great Time War, the Master degenerated back into his previous incarnation, the War Master, after being caught in a time paradox. (COMIC: Fast Asleep)

The War Master would eventually regenerate once again, this time into the Saxon Master after escaping the Time War. (TV: Utopia)