Book 1 of 2, where Superman finds himself poisoned by Lex Luthor. And shockingly, the Man of Steel is dying. He then embarks on a plan to ensure there is continuity in protection for Earth, after he is gone. Before that, it’s prophesised that his time on Earth won’t be over until he completes 12 feats of greatness.
This Wikipedia entry has more details. But do read this excellent, very human, Superman story first.