Complex sentence: Unlike Saruman, Denethor was too strong to be corrupted by Sauron's lies, but it served Sauron well enough for the "palantír" to show Denethor the truth of the vast forces arrayed against him.
Split into simpler sentences: Unlike Saruman, Denethor was too great to be corrupted by Sauron's lies. However, it served Sauron well enough for the "palantír" to show Denethor the truth of the vast forces arrayed against him.