The piece you are looking for is titled (track 22) from the Shiki Original Soundtrack, composed by Yasuharu Takanashi .
While "SHI-KI" is the main title track often associated with the series' dark and emotional atmosphere, several other tracks from the same album are frequently labeled as "sad" or "emotional" in various music collections and playlists: Yasuharu Takanashi — Shi Ki Sad Emotional Music
: A mournful piece (track 23) that serves as a concluding emotional theme for the soundtrack. Shiki Original Soundtrack | SVWC-7768 - VGMdb The piece you are looking for is titled
: A highly popular emotional and beautiful track (track 2) often cited as one of the most moving pieces in the series. Yasuharu Takanashi — Shi Ki Sad Emotional Music