Final Fantasy IV Official Art
In my opinion, some of the darker art in the Final Fantasy series comes from Final Fantasy IV. Cecil's dark form is very dark indeed, as you will see in this official art from Yoshitaka Amano. Notice as you browse through this gallery just how accurately the sprite artists from Final Fantasy IV translated some of Amano's art. Thanks to FF Compendium, and Shinryuu's Lair for each providing some of these scans.