Dormammu

Dormammu is a powerful extra-dimensional villain from Marvel most commonly associated with Dr Strange - being one of the Sorcerer Supreme's most reoccuring and powerful enemies, though he has appeared in other titles as well (even showing up, albeit in a much more kid-friendly fashion, in the Super Hero Squad cartoon series).

Nature
Established from early on as a warlord from an evil dimension labelled the Dark Dimension he is a creature that seeks universal conquest and has identified Dr Strange as one of the most prominent threats to that goal, for this reason Dormammu has tried many times to destroy the mystic hero but despite his vast power has always failed to do so.

Dormammu often has himself to blame for these defeats however as he often allows his arrogance to get the better of him and he can be somewhat easily tricked by people that know what they are dealing with - of course these flaws may also be the only real things preventing Dormammu from taking over all of reality, given the amount of power he has at hand.

Villains associated with the Dread Dormammu include Baron Mordo, an evil wizard and archenemy of Dr Strange, Umar - his sister and Sattanish, a powerful demon-lord created by Dormammu as an extension of himself - all of these beings have faced Dr Strange on regular occasions and are considerably powerful yet dwarved by Dormammu himself (even Umar, who is of the same species as Dormammu, is unable to match her brother in raw mystical-power).

Powers/Abilities
Dormammu is near godlike in scale, capable of taking on some of the most powerful beings in Marvel continuity and surviving, his power was even great enough to spawn a sentient being such as Satannish and his entire form is made out of raw magical-energy, only serving to boost his considerable power further - he is an object of worship by thousands, if not millions, and the worship he recieves also fuels his power and as such he could be considered the most powerful opponent Dr Strange has faced.