The BBC has released a climactic promo image that features Gomez, Simm, and Capaldi marching to war.
The Master was famously played by Roger Delgado from 1971 until his death in 1973, but similar to the Doctor, the character has undergone several regenerations over the years.
Fans of the rebooted era of the show are promised a chilling return for the man behind the Master and an unexpected end to Season 10.
The last time we saw Simm, his Master briefly became Prime Minister, was killed off, and then later resurrected to create a race of his own clones.