@Meddler: What would you do in designing a Dark Knight?
Celes, the Dark Crusader
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Kassadin is about as close as we get to the "Dark Knight" trope, but whenever I think "Dark Knight", I think Cecil from Final Fantasy IV or Guts from Berserk. Basically, good people who use dark power for good causes, often at severe personal cost. Kassadin's gameplay doesn't really reflect this, though; he's simply the mage-killer assassin.
Would it be possible to delve deeper into Dark Knight mechanics (of sorts) for a future Champion? If so, how would you go about it?