Fifteenth Master

The Fifteenth Master is the fifteenth incarnation of the Time Lord known as the Master. After being exterminated and stealing a new body in his fourteenth incarnation, the Master regenerated and was held prisoner by the Time Lords.

He then was released because the Time Lords knew he'd be the perfect warrior for the Last Great Time War. To ensure he wouldn't fly off again, they destroyed his TARDIS and made him wear a ring he couldn't remove. This ring would kill him if he left Gallifrey's time zone.