And finished.
http://www.youtube.com/watch?v=lh_mUVcZzWc
Here we have Eclipse, a dark knight. I love armor suits and I totally had fun designing his armor; not too overly done, but it gets the point across while fitting in with the cutesy cast. Normal dark knights would look a little less detailed than Eclipse, and I'm not sure if I'll make a paladin equivalent character that the party would interact with.

Very rough ideas on a story:
While paladins have the laws of their kingdom to abide to, dark knights have their own creed that they follow. Knight would cross paths with Eclipse on several occasions where they'd talk and exchange opinions on various matters, or sometimes Knight would have to decide on stopping Eclipse's actions should he see them as the wrong, and other times he'd be challenged to a "friendly" duel. Eclipse is overall a good guy, but needs some sense knocked into him at times, unfortunately Knight is usually the one taking the beating. Knight would eventually learn some dark knight skills through Eclipse's teachings.

In a video game perspective, I'd have no idea on how to play this out. The story and levels would be the same no matter which character you play, least that's how a 2D side-scrolling hack'n'slash should be, but maybe if the leader of the party (the game host) was a certain character, the story would be slightly altered for the character they're playing as, and maybe even do some of the areas in a different order. Otherwise, I can just have Eclipse have beef with the whole party all the same and not really change the story/dialogue too much. I could just picture a Mage solo run right now, and she immediately begins to nuke him with spells right after a dialogue sequence.