"Abyss Curtain", also known as "Their Mom's Labia".
Also, great, new slaves appeared for mc to turn into cute girls.
Wonder if they realise, they're in the presence of the demon lord.
I'm having a déjà vu, there was another story with goddess/another enslaved summoned hero, where he met another summoned and he acts all cringey chuuni... Fabiniku, maybe?