When the first set pics from Doctor Strange hit the interwebs last month, a lot of speculation swirled around who Mads Mikkelsen was playing. While a lot of people were guessing that he was playing Dormammu, but then it was announced his character's name is Kaecilius. Unless you're steeped in the knowledge of every side character from Marvel's Silver Age, you likely have no clue who that is, but he is a legit Doctor Strange villain, albeit one who didn't do much in the comics.
Moviepilot has put together a hell of an article about the character, and I wholeheartedly recommend you read the full article, but if you don't have the time, here are the basics...
In issue 130 of Strange Tales, released in March 1965, Kaecilius was a minion of Baron Mordo—apparently an ally of Strange's in the film, as played by Chiwetel Ejiofor. In the following issue, Kaecilius traveled to Hong Kong to round up a band of smugglers that will help him track down the good doctor. Unfortunately, they clown him into submission
The character popped up in a few more issues before making his final appearance in issue 143. He's honestly no more than a footnote in Marvel lore, which means he'll be just another Marvel villain of the week in the film. I imagine the rift between Strange and Mordo will drive the latter into being the antagonist in the sequel, but first we've got some awesome Mads Mikkelsen action to get through.
Again, please go read the full article for more info, and get ready for Doctor Strange's release on November 4!