The Dread Dormammu's current form was born when, after being exiled to the Dark Dimension, he conquered his new home and created a new body for himself composed entirely of magical energy. After his first invasion attempt on Earth was halted by Dr. Strange, Dormammu became one of Strange's most powerful recurring enemies and has frequently attempted to conquer Earth and kill Dr. Strange.
Dormammu's greatest weakness has historically been his sense of honor. Frequently after being defeated he will swear an oath to never again repeat his acts -- which only leads him to scheme a new attack that does not violate the rules of his previous oaths. One is advised to never make a deal with Dormammu, as Earth's greatest lawyers envy his ability to cheat a contract.
In battle, Dormammu possesses an immeasurable amount of magical power, and has been shown using it in an endless amount of ways. Energy blasts, magical shields, and teleportation are only the most basic tools in his arsenal. When fighting indirectly, Dormammu's bottomless army of The Mindless Ones can swarm an enemy and overcome them quickly with brute force and sheer numbers.