The Dreaded One
If the powerful Dormammu’s exact beginning has ever been recorded on Earth, it was either lost or remains undiscovered. But, suffice to say, the powerful being has ruled his native Dark Dimension for untold ages and is its complete master. Despite his dominion over his home, the evil entity still craves more power and has decided that only by conquering and subjugating all other realms, planes, and dimensions can he ever truly be sated in his desires.
For reasons purely his own, Dormammu has set his eye on Earth’s plane as the most important to conquer, perhaps owing to its suffusion of magic and magic-wielders. At some point in Earth’s past, a tome—the Book of Cagliostro—was crafted to include a ritual to summon Dormammu, providing the means for humans to approach the powerful being and form a connection between Earth and the Dark Dimension. This is important because the presence of three magic Sanctums on the planet prevent Dormammu from invading himself, despite his immense abilities.
Power has its Limits
In the timeless Dark Dimension, Dormammu reigns supreme and wields forces beyond human comprehension, but he can be impacted by powerful artifacts such as the Infinity Stones, even to the point of cowing him and halting his actions.
In addition, the entity can grant a portion of his power to humans and other lesser beings for the purposes of extending their mortal lives and a certain degree of magical proficiency. The exact cost of these gifts from Dormammu varies, but can range from physical disfigurements to deeper disabilities.