The Demon is the secondary antagonist of the film, Rock and Rule. It is the master of the villain, Mok Swagger.
The Demon was imprisoned in another dimension, and Mok sought to free it to give him immortality, legendary fame and ultimate singing. Mok tried several times throughout the film to unleash the Demon, all with disastrous consequences.
It was only when Mok realized he needed an overload of power that he managed to create a gate to the Demon's dimension.
The Demon came out of the gate in a burst of nuclear orange energy and tried to devour Angel, the victim Mok had chosen. But Omar and the rest of his band appeared.
Together, Omar and his band fought the Demon, eventually driving it back with Angel's powerful singing Voice. However, a terrified Mok attempted to thwart their efforts, realizing the computer's prophecy about "no one" meant that more than one being was needed to stop the Demon. Mok tried to kill Omar, but Zip, one of his henchmen, distracted him by kicking him into the portal to Hell. As the Demon got sucked inside from Angel's power, Mok also fell in, after crying out to be remembered as a god.
Little is known of the Demon's personality or even its real name. However, it was awesome, as shown by how both Omar's band and Mok's henchmen react upon seeing the creature released. It seemed to be able to grant wishes - thus probably making it a type of Djinn - or perhaps even another wish-granting demon. It could only vocalize in growls, grunts and screams, and this might have been due to it being released, it had yet to regain full power. But it could consume living matter, and sought destruction. Mok was eager to meet the Demon yet he was, ironically, totally ignored by the beast, who sought to seek vengeance on its imprisonment and consume as much as it could.
It is unknown what demon it is meant to be, yet, given the film's mention of Lot and the city of Sodom, it is probably meant to be Azazel, a fallen angel and collector of souls who is an agent of the Devil and who all fear.