Complex sentence: The episode is the thirteenth Christmas special since the show's revival in 2005, and is Capaldi's fourth and final Christmas special as the Twelfth Doctor.
Split into simpler sentences: The episode is the thirteenth Christmas special since the show's revival in 2005. It features the final regular appearance of Peter Capaldi as the Twelfth Doctor, and is Capaldi's fourth Christmas special as the Twelfth Doctor.