We know that Doctor Strange will be an origin story. And we know that, by the end of the movie, Baron Mordo will be revealed to be a villain. However, up until now, we didn't know who the movie’s overall main antagonist would be. But now, thanks to Diamond Select's Doctor Strange play figures, we may have figured that out. Since the toys aren't always a reliable source, you might want to take this with a grain of salt.
According to Heroic Hollywood, Mads Mikkelsen's mysterious character is Kaecilius, as it was rumored before. This is a minor comic book character who was Mordo's disciple and errand boy. As far as the Doctor Strange villains are concerned, he is a very low tier character. And this gives Marvel the opportunity to reshape him to fit in with the MCU.
To be noted, the toys' description pairs the characters as follows: Doctor Strange with The Ancient One and Mordo with the newly revealed villain.
Not the most reliable source – but here goes. (CBM)
This new piece of information can be paired with an older rumor about Mordo's story arc. Just like in the comics, Kaecilius will be presented as one of Mordo's former pupils. However, unbeknownst to everybody else, they are still working together. So, when Kaecilius finally opens a portal toward a dark dimension, he isn’t doing it for himself.
As said, this is just a rumor. However, starting with a minor villain is the right thing for Marvel to do with Doctor Strange. With Mordo being revealed in the end as a villain, a sequel will already have found its villain. And, if everything until this point is confirmed, from Mordo to Dormammu won't be such a long way.
Again, this is all speculation, based on a line of toys. And the toys aren't always the most reliable sources of information.
Doctor Strange (Wikipedia)
