for whom
Archaic literary expression meaning 'for whom'. In modern Japanese, it is mostly encountered in titles, poetry, or fixed phrases, such as the famous novel title 誰がために鐘は鳴る (For Whom the Bell Tolls).
'For Whom the Bell Tolls' is the title of a Hemingway novel.
For whom is this song being sung, I wonder?
Standard kanji-kana mixed spelling; the kana がために is part of the fixed expression.
Archaic kanji spelling using 為; rarely used in modern writing.
Classical Japanese construction: 誰 (who) + が (possessive/archaic subject particle) + 為 (ため, sake/benefit) + に (particle). The が here is an archaic possessive or subject marker, not the modern conjunctive particle.