Shadow lord has one of the most cliche and overused back stories in fantasy gaming/literature, while final fantasy games very rarely stray from well trod ground and never push the envelope storywise, even his name is cliche and silly. Shadow Lord? Really? That sounds like a working name that would be changed prior to release.
Darnus, while really just as cliche ( again FF storytelling is built on them), is refreshingly a lesser used one in fantasy tropes. He also has a decent name.
edit: he's also coming off as "Mid-Boss" at best. Shadow Lord was a "Big-Bad" with a cliche story, Darnus is being implied as being a villian with a cliche story and "a pawn...OF A FAR GREATER EVIL!!(dun dun DUNNNNN! *dramatic re-verb, ominous crash of thunderrrr....*
