After the master had fallen into the eye of Harmony the power of the eye of harmony stripped away the human body of Bruce that he had possessed, transforming him back into the husk like creature he had been after running out of regenerations. The decaying master died in the eye of Harmony. The master as a ghost (that resembled the Roger Delgado master) was then found by the time lords. They told him he would be needed for the time war. The master had no choice so he made a deal with them that if he agreed to fight in the time war they'd give him a new body. So the master was given a new body and became a time lord again with a new regenerative cycle. This new body was this incarnation.