The 4th arc of the Re:Zero kara Hajimeru Isekai Seikatsu manga.

On to the next stage: the witch’s tomb known as the Sanctuary- Determined to release Rem from a mysterious slumber, Subaru and Emilia return to Roswaal’s manor in search of clues on how to reach the Sanctuary, the secret location to which Ram and Roswaal escaped. But the way there is fraught with the unexpected-especially for Subaru, who finds himself in the company of a curious figure. A new chapter begins, and with it, a new adventure…

(Source: Yen Press)