If making something that would make a Dark Knight feel like you're playing a Dark Knight, more focus on HP drain effects, debuffs that lower enemies damage dealt, and HP shields make Dark Knight feel more like a magic tank on top of existing mitigation, and more focus on offensive GCDs with either unique recast timers or high cost to use and damage over time effects with Dark Arts having stacks for you to use ignore high cost but generating it can be done with a high cost GCD(no cost if outside of combat), reduce Darkside's duration to 30 seconds while making it's re-cast timer 25 seconds(bring the button that activates with having to do damage first) and buff Blood Weapon to 30 seconds and have it affect all damage dealt and be attack speed increase but make it's re-cast 25 seconds(it would be more of maintenance buff like Darkside).

That's the general way I would go about it building Dark Knight as a class...