Baron Mordo — Baron Mordo started as Strange’s competition to become an apprentice to the Ancient One. Because of jealousy, Mordo eventually betrayed the duo leading Strange to take the position and in time become the Sorcerer Supreme of Earth. Mordo led a life of crime as a regular foe of Strange but later turned over a new leaf and fought a cancer diagnosis.
Dormammu — The dread lord has been a longtime adversary of Strange. Dormammu rules over the Dark Dimension, which he conquered by force and by overthrowing the established leader. He has challenged Strange many times in an attempt to take over the Earth realm but has lost. He is a being of immense power and a master of chaotic magic.
Mephisto — Basically, Mephisto can pose as the Devil, but he is not Satan and does not rule over Hell. He is a ruler of a hell dimension and has established himself as a broker of souls. Mephisto, father of Marvel stalwart Blackheart, is responsible for the the creation of Ghost Rider.
Nightmare — Nightmare rules over the Nightmare World in the Dimension of Dreams. He is capable of manipulating the dreams of humans and gaining control over them. Nightmare is a demon attempting to take over the realm of the awake.
Shuma-Gorath — Shuma-Gorath is one of a race of creatures — known collectively as Shuma-Gorath — that take the shape of a cycloptic tentacled eye. In Strange’s time, Shuma-Gorath attempts to enter the mainstream dimension through the mind of the Ancient One. This results in Strange having to kill his master to prevent Shuma-Gorath’s manifestation. Shuma-Gorath again tries after Strange absorbs him and begins to turn into him. It is eventually killed but resurrects itself through Chaos Magic.