Elysia, the ethereal paradise. For generations, the people of the realm has battled monsters and demons of all kinds, invaders from across time-space who seek nothing else but to devour all that is good in the world. With their mastery of the arcane, the mages and paladins of Elysia had struggled to hold the line. But in a desperate attempt to turn the tide his favour, the dastardly demon kings resorted to sapping the world's magic for his own ends. Bereft of power, and helpless in the face of destruction, the princess of Elysia summoned the last of her magic to call forth heroes of another kind, or rather, heroines of the arcane kind.
You are a magical girl, maidens of extraordinary power, dedicated to defending your homeworld of Earth (or the previous world you had been summoned to) from evil. Called upon by the mysterious princess to save Elysia from the Four Demon Kings and the Melodia Cult, you find yourself locked with your interdimensional comrades in a struggle for survival. However, despite its idyllic appearance, Elysia hides secrets that nag at your innate suspicions. As some magical girls know firsthand, nothing is ever as it seems.
Elysia is colonized by magic-using humans from another dimension. Since magic is not native to Elysia, the humans resort to wormhole displacement (otherwise known as summoning) and genetic engineering to flood Elysia with creatures and people able to generate magic on their own, such as elves, dwarves etc.)
In order to meet the demands of the burgeoning population, and to control and unite the population, the human king summoned demons from another dimension to act as artificial threats, with heroes slaying the demons and harvesting their mana.
As time went on, heroes who had harvested a large amount of mana soon grew too powerful for the royal family to control, leading to multiple usurpations, civil wars and even turning coat to become demon kings. The current ruling family, themselves descended from one such hero, sought to prevent further turmoil by summoning Terrans as heroes, who'd be far less likely to rally a support base among the people, and more easily controlled. Those that become too powerful could then be sacrificed for mana on the promise of returning home, or married into the family on various privileges if persuaded.
When dealing with the previous demon lord (himself a former Elysian hero who turned coat), a Terran hero team was summoned, composed of archetypes of various genres: An Isekai hero paladin, a Shounen protagonist combat monk, a Military/Mecha ranger, a Harem protagonist mage, an Idol thief and bard, and a magical girl priestess. Each of them came from different versions of Japan, leading to tension between the various heroes.
Kazuki Amamiya - Isekai Protagonist, from a timeline closest to our own, though with Bush-era universal healthcare.
Kentarou Azuma - Shounen Protagonist, from a post-apocalyptic timeoine where Able Archer resulted in WWIII.
Gai Mukuro - Mecha/Military Protagonist, from a cyberpunk timeline where the Lost Decade never happened, the Cold War never ended, and Japan is divided between a communist North and capitalist South, similar to Korea.
Nobuyuki Takamura - Harem protagonist, from a timeline where Nationalist China joined the Axis, and Japan joining the Allies, allowing it to control large swathes of China and retain its imperialist ambitions.
Mei/May Mirai - Born in a timeline where MacArthur removed the Emperor, leading to a Japanese republic and greatly liberalized and Westernized society.
Kohaku Amamiya - Kazuki's sister, Mahou Shoujo fan.
Slaying the last demon lord, the team returned to accept their rewards. Nobuyuki, Gai and Kentarou all elected to stay, while Kazuki, Kohaku and May elected to return. Unbeknownst to the team, the queen intended to absorb the mana of the team. May reluctantly lets Kohaku accept the 'reward' first, a decision that would come to haunt her. Sneaking a peek into the Queen's chamber, she could only watch in horror as the Queen tore out Kohaku's heart, killing her and devouring the organ to replenish her power. Warning the others, the team fled, with Nobuyuki still in denial of what happened. Becoming Demon Lords, Kazuki's surviving teammates had since conquered large sections of Elysia, fighting amongst themselves over grievances each had for the others. Kazuki, in turn, isolated himself in grief, though the Queen also branded him a Demon Lord as well, while plotting to avenge his sister.
Angered by the sudden disruptions in her plan, the Queen sought to summon more heroes to battle the rebelling heroes, but with only a fraction of energy to spare, and highly distrustful of the usual Terran heroes sent to do her work, she decided for a radically new approach. Taking a page off the girl she had sacrificed, she chose timelines where Terra is protected by powerful magical girls. This had a dual function: it allowed her to avoid investing magical power on new heroes as she normally would, and she could harvest the new heroines for more mana when their job is done. With that in mind, she prepared to bring forth the mightiest heroines Terra has ever known.