The mysterious girl that showed up out of nowhere was actually Filo. She's a filolial queen with an advanced transformation ability, and she can even take human form thanks to it. However, her clothes get ripped every time she transforms, meaning Naofumi has to get her some magic clothes if he doesn't want to go broke. The party leaves on a new adventure to gather the materials required.
It's because you're converting 
your mana into thread.
هذا لأنّكِ تحوّلين المانا خاصّتك إلى الخيط
Es macht ja auch Garn
aus deiner magischen Kraft.
Porque creas el hilo
usando tu poder mágico.
Porque creas el hilo
usando tu poder mágico.
Normal,
tu transformes ton mana en fil.
Perché stai convertendo
il tuo potere magico in quel filo.
É porque você está
transformando sua mana em fio.
Всё оттого, что твоя мана
превращается в нити.