The Demon Army is a powerful military group, organized and led by King Satan and his War Generals. The army was conceived as both the protectors and invasion force of the Demon World.
Created from the culmination of several different Demon Tribes, the army consisted of several kinds of demons of all shapes and sizes. Due to the large variation among the many factions, the types of magic and methods of combat differed greatly between individuals. While some such as the Malebranche Legion led by Malacoda were known to favor the use of magic, many others were shown to wield common weapons such as daggers and clubs, showing a large variation in their preferred method of attack. The army appears to be split among the Four War Generals, with each commanding a sizeable legion of creatures. Whether due to its leadership or not, the group as a whole appears to be well-organized and powerful, easily spreading across and subjugating all five continents of Ente Isla. The Demon Army was given free reign to destroy the human resistance by any measure necessary, and they did so by brutally murdering anyone who stood in their way, lighting fields and villages ablaze, and slaughtering the innocent. Their acts of cruelty resulted in a widespread panic and fear of the monstrous military.
The Demon Army faced resistance at the hands of the Church of Ente Isla, who trained Emilia Justina as a Hero, allowing her to wield the Sacred Sword - Better Half. The Hero joined with the Allied Knights Order, and together they drove back the evil hoard. The strength of the Demon Army diminished greatly due to the acts of Emilia, who defeated their leaders one after the next. She managed to defeat Lucifer in the West, killed Adramelech in the North, forced Alciel to retreat in the East, and slayed Malacoda on the Southern Continent. With the help of Emeralda, Albert, and the High Priest Olba, she proceeded to attack Demon King's Castle, engaging their leader in a fierce battle in which she overwhelmed the dark ruler, eventually slicing off one of his horns. Angered by his defeat, Satan declared he would leave Ente Isla in the hands of the humans, vowing one day to return and conquer it. Opening a Gate in the sky, he fled to Earth with Alciel.
Following the defeat of King Satan, the Demon Army was left in a dire situation, having lost many soldiers along with their beloved leader and his four generals. The remainder of the military returned to the Demon World in failure and fell under the command of Kameo, the Bird Demon who served as the Demon King's minister of foreign affairs. Known for his intelligence and tactfulness, the minister ruled on behalf of his missing leader while searching for his whereabouts. Malacoda's former assistant, Ciriatto, aided him in these efforts, gathering up the remains of the Malebranche Legion in a mission to search Earth for answers.
While Kameo is successful in passing though the Gate, he is weakened upon reaching Japan. Luckily he is discovered by Sadao Maou and Shirō Ashiya, though he is initially unable to recognize them as his superiors and mistakes them for humans. The situation is cleared up when he sees Hanzō Urushihara, whose face remains unchanged in human form. Due to his loss of power, the Bird Demon finds himself in the form of a small black bird, his size changing depending on how much power he can amass. Several days later, they are joined by the Malebranche Legion, who plans to attack Japan, but they are stopped by Emi Yusa, who reveals herself as the Hero Emilia and repels the 1,200 demons long enough for Maou to arrive with Kameo and the others. Due to the strong magic leaking from the open portal, the Demon Leaders manage to regain enough to power to resume their true forms. Declaring he is still alive, King Satan then orders the Demon Army to return to Ente Isla bearing news of his continued survival, the others all bowing before him in respect as they heed his command. Kameo also returns to Ente Isla to pass on this message, willing to resume his leadership of the Demon World until the true king is willing to come home.