The magical girls fought on humankind's behalf when a demon army invaded.
Tragically, they weren't strong enough to save us, and Earth became Hell's dominion.

However, not all the magical girls are gone.
Serina, Yui and Shion continue to fight the good fight.

Demon lord Grace hasn't made it easy.
The girls are cursed with a demon lust that weakens them from the inside.
Then, Grace gives all minions an incentive:

Demon lord: "Catch those girls and you can f*ck them to oblivion!"

Magical girl: "No... not like this... not by these repelling things... I, I......

I don't want to cum! NnnnnnaaaaaAAAAA!"

[From DLsite English]