War Master's second regeneration

After opening the chameleon arch, Professor Yana was reminded of his true identity as the War Master. He then began sabotaging his Utopia Project that was meant to save the human species. He then killed his assistant Chantho. As she lay dying she shot him.

The War Master then stumbled into the Doctor's TARDIS and locked it. Deciding that he would become "young and strong" to match the Tenth Doctor, the War Master regenerated into the Saxon Master, who screamed as his form emerged. (TV: Utopia)

Behind the scenes
The Master's regeneration in Utopia was represented by a Battles in Time trading card.