To prove one’s worth
As Dahlia mulled over everything the plant woman had told her, she came to a conclusion that made her somewhat uncomfortable. Nothing Euryale had said had come across as a lie or deception and, assuming it was all true, Dahlia really didn't have a reason to refuse. She never truly cared for the afterlife and becoming an outsider or servant of the gods. She knew that degrading her soul too much could negatively affect her health and casting, but beyond that she couldn't really bring herself to care what other price she might pay.
To her, the afterlife may as well have been a writeoff from the very beginning. Perhaps if she was religious or used faith based magic like Lilli, that might have been enough to make Dahlia refuse Euryale's offer, but that was the thing: she wasn't religious in the slightest. In the end, the girl just couldn't find fault with just going along and accepting Euryale's request but that, in of itself, made her uneasy.