"The Sickness ___ Death" (Kierkegaard book) (4)
I believe the answer is:
unto
(Other definitions for unto that I've seen before include "'Do ... others, etc.'" , "Biblical preposition, the archaic 'to'" , "'''For ... us a child is born'' (Handel's Messiah) (4)'" , "To (obs.)" , "'The Lord said . . . . Moses'" .)