Introduced to the Marvel Cinematic Universe in Doctor Strange, Dormammu is one of the most powerful mystic forces in the Marvel multiverse. A dimensional conquering force, he is the lord of Dark Realm and the brother of his equally powerful sister and rival, Umar. Doctor Strange often has to push himself to the limit to defeat Dormammu and protect the Marvel Universe.
And as shown in one 2005 Defenders story, Doctor Strange failed in that mission -- resulting in the demented and demonic Dormammu-World.
In Defenders #1 by Keith Giffen, J.M. DeMatteis and Kevin Maguire, Dormammu decided he was finally tired of contending with the likes of Doctor Strange and instead elected to forge an alliance with Umar. The pair arranged a plan to strike preemptively at Earth-616 before they had time to respond -- with Doctor Strange only learning about the attack thanks to Nightmare, who feared a reality controlled by Dormammu that would ban dreaming of all kinds and cut him off from his power. Doctor Strange quickly tried to assemble the Defenders (only recruiting Namor and the Hulk) so they could respond to the threat before it overtook the world -- unwittingly playing right into Dormammu's hands, with the villain lying in wait alongside Umar and a dark army poised to overwhelm the heroes.
Defeating Strange and capturing him alongside Namor, Doramamu used the stolen Eye of Agamotto to help usher in the Age of Dormammu -- by combining his full might with Umar and slaying Eternity. Gaining his power, Dormammu remade the Marvel Universe in his image -- creating the Dormammu-Verse. The resulting corruption effectively turns all living beings on Earth into demonic versions of themselves -- including the Avengers, now transformed into his fiendish Hellriders. their massive ranks included demented versions of Captain America, Daredevil, Spider-Man, Iron Man, Wolverine, Beast, and Thor, serving Dormammu's demented purpose. Dormammu even created his own duplicate of Doctor Strange to serve at his side, who in turn worked as the feared leader of the Hellriders.
On top of their enhanced abilities and corrupted mindsets making them extremely ruthless, each of the Dormammu versions of the Marvel Heroes was amplified to be more or less unkillable. The only person on Earth who seemed to fully remember what had happened to the world was Clea, Umar's daughter and Doctor Strange's former pupil and love interest. But she remained a captive of Dormammu, a weapon to be used against the true Doctor Strange. Thanks to being in the Dark Dimension when Dormammu rewrote reality, Hulk, Namor, and Doctor Strange are spared being recreated in Dormammu's image. However, Hulk is separated from the others and ends up in the possession of Umar -- who takes a liking to the Hulk and attempts to torture Bruce Banner until he reverts to that form.
However, it's eventually revealed that Umar is working with small resistance to Doramammu's rule, led by this world's version of Namor. After the core] Marvel Universe Doctor Strange is captured, Namor unites with his alternate self and fights alongside him and Umar to shatter the control Dormammu has over the world. By the time Namor attacks, Doctor Strange is revealed to have faked his captured and disguised himself as his corrupted counterpart -- helping give the resistance and Umar the opening they need to attack and siphon Dormammu's power. Luckily, the heroes were able to overcome this darkness and restore the universe before Dormammu was able to fully cement his demented dark realm.
This is a good indicator of the kind of darkness that would be made the law if Dormammu was able to ever fully defeat Doctor Strange and truly take over the world. For all the heroism in the world, there is little even the likes of strong hearts like Steve Rogers or the might of Thor can prevent.