No one knew how exactly the gates of hell came to be thrown open. Perhaps it was human scientists, conducting experiments that toyed with powers best left alone, or a hidden coven of sorcerers who had survived into the modern day, seeking vengeance on the world that had forced them into hiding. Or perhaps it was the demons themselves who tore open a hole in reality, seeking new prey. Perhaps it was simply the ordained time. But in any case, the results were clear. A breach between worlds was opened, the legions of demons poured through.
Their numbers seemed limitless, their blood lust insatiable. The army tried to stand against them, but there didn't seem to be much that tanks and missiles could do against the dark magics of the underworld. And the demons came in many different types. There were the imps, small and mischievous, and the brutish, heavily armed soldier demons who formed the bulk of Hell's armies. There were the incubi and succubi, sultry and seductive, and the eldritch demon wizards, mysterious and cunning. There were the savage hellhounds, and the great dragons that rained down fire from the skies. Presiding over them all were the mighty demon lords. And there were rumors of other types besides these.
It didn't take long for society to start breaking down as demons overran the world. Some people they killed - and those they killed were often eaten. Others seemed to fall into a sort of trance, filled with awe at the demons and desiring only to serve and worship them. And sometimes, a demon might make... friendlier contact with a mortal. Sometimes, especially for succubi or incubi, this involved sex, or even just a kiss - other times, it might take the form of a friendly touch or embrace. But in any case, the human in question found themselves transformed into a demon themselves, of the same type but opposite gender of the one that turned them. Those transformed retained their human minds and memories, but gained demonic powers and instincts - it was possible to resist them, for a time, but in the end almost everyone succumbed eventually.