For anyone not familiar with Monster Girl Encyclopedia, the setting is your typical world of swords and sorcery. Since the beginning of the world, the Order, led by the Chief God of Light and her chosen Heroes, fights back against the Demon Lord and his armies of monsters. Except, somewhere along the way, one strong hero joined forces with an aspiring succubus named Lilith, who usurped the previous Demon Lord with his help. The newly crowned Demon Lord then cast a spell that turned all the monsters into Monster Girls. Now, Monster Girls claim husbands instead of taking lives. Unfortunately, Monster Girls can only give birth to more Monster Girls, so humanity must keep fighting to avoid the extinction of both sides while Lilith bides her time, preparing for her second grand spell.

On to what's different from the original setting.

Roughly a millennium and a half after Lilith became Demon Lord, she has managed to cast her second grand spell, allowing Monster Girls to give birth to human males. As a result, society has advanced significantly, with features like better plumbing, architecture, and city planning popping up. With the help of magitech, one could consider their technology to be almost on par with Earth's, at least on the Monster Girl side. However, humanity continues to flounder technologically, and they are stuck in the dark ages. Humans visiting demon realms became increasingly common, before stopping altogether due to the lack of humans outside Monster Girl society. Well, almost. Only the radically anti-monster faction of the Order remains, desperately struggling to keep the old ways of humanity alive.

One fateful day, a young child begins to cross the street, oblivious to the solid red hand shining at him from across the sidewalk. A young man; the child's teacher, in fact; notices and rushes out to pull him back to the safety of the sidewalk. It's too late, however. A semi is coming toward them at the speed limit, and it's all the young man can do to shove the child out of harm's way back towards the sidewalk. His death is merciful, nearly painless in how quick it is. The young child will grow up haunted by that day, but that is a story for another time.

Moments later, realms away from Earth, on a planet known as Eos, an infant cries out. A lonely soul, stripped too soon of its greyscale life, has been given another chance. But not even the Fates can predict the consequences of repurposing unused thread; consequences that will span across nations and even dimensions...