Mad's Mikkelsen Kaecilius is not the only villain in the film 'Doctor Strange'. The secondary villain in the film is Dread Dormammu who summoned by Mad's Mikkelsen Kaecilius. Dormammu plays important role in the film's final act but no actor is listed playing the part in the closing credit.           

When the director of the movie, Scott Derrickson was asked who played the second villain, Dread Dormammu, he does confirm that Benedict Cumberbatch is part of the equation, bringing the iconic Marvel villain to life. Derrickson revealed that the voice used in portraying Dormammu was a British but didn't know who or even the name and then Benedict did the facial capture.

Another Marvel film; Avengers, where one of their grand villain, Thanos, that at the end of the movie there hasn't any actor assigned in the role. Not until two years after and revealed that Josh Brolin was the actor who plays the role of the mad titan.

According to IGN, Derrickson was asked if this was the way to defer casting the role, the director said: "that wasn't the case." According to him, it wasn't that at all. They knew that it was an all CG character, as they were in production Benedict brought up an idea and said 'What would you think about me doing it?', the director then replied: "Let me think about it."

Derrickson revealed that the more he thought about it the more he liked the idea because no one understood the character of Dormammu better than Benedict did. He wrote that role to be an ultra-inflated version of Strange.There is something interesting about the confrontation of this tiny, little guy who has the power of time and monstrous conqueror who trapped by a clever gambit. The directed then added that nobody can interact with Benedict better than himself, via Screen Rant.

The dual performance was all Cumberbatch's idea. He plays both hero and a villain in the film "Doctor Strange" .