Who he is: Dormammu, the Dread One, master of the Dark Dimension
His power: He can do pretty much anything in the Dark Dimension, but he requires his zealots in the proper MCU reality to make contact with him before he can reach into other worlds. Luckily, he can also grant special powers to his followers in the other reality, so he has ways of getting around.
His story: Dormammu first appears in Doctor Strange, and as an ancient, all-powerful cosmic entity, he doesn’t have much of a backstory. What we do know is that he really wants to conquer the entire multiverse, and he’s trying to break into the MCU reality by offering power to sorcerers. Thanks to Kaecilius, Dormammu actually came pretty close to destroying the Earth, but he was defeated by Stephen Strange’s newfound selflessness and his own crippling impatience. Strange used the Eye Of Agamotto to create a time loop where he would constantly die and be reborn, taunting Dormammu with every chance he got until the Dread One simply gave up.
Played by: Benedict Cumberbatch, via voice over and facial capture.
Currently, Dormammu is: Hanging out in the Dark Dimension, presumably figuring out how to take things slow and appreciate the little things in life.
Where will we see him next? Unknown, maybe never.