User blog comment:FallenShadow/Fallen's Reviews: Chapter 341-343/@comment-15382134-20130711221848/@comment-33019320-20130711224234

I believe when she mentioned underworld, she was referring to the fact that she was working in a Dark Guild now.

Besides, even though the name Tartaros refers to the underworld, we still don't know if that's actually their theme.