Dormammu is another one of those dudes from a far off distant dimension. We're talking about the type of place where he was pretty much just a big ball of light. Eventually he took a human form when he entered another dimension with humanoids. He's not even in the Earth dimension by this part of the story. In fact, there's a whole lot of otherworldly stuff to his story before he even messes with our big blue planet.
You may remember Dormammu from the 'Doctor Strange' movie as the main villain (sort of). He's the main villain but he's only in one final battle. He beats Doctor Strange multiple times, which is kind of cool to see, but he doesn't win. Doctor Strange uses the time stone to reverse time so that Dormammu can never actually win.