I think thats where the soul lies. or what they call heaven
what deadpopito said, and i think she wants to meet himmel's soul
Ya what the others said. She felt regret from not connecting more with Himmel and Eisen recommends she talks to his soul. I think Eisen pities Himmel for loving Frieren.
http://www.mangago.zone/read-manga/sousou_no_frieren/uu/mpn_chapter-2324280/pg-12/
I think that there is some kind of connection to the afterlife.
Why are they going to Demon King's castle again?