Terror Blade combo - This would be the Drks main 1, 2, 3. The 3rd hit would do the typical increase enmity, however the FIRST hit would simply add a stack of Terror to the target. Terror would have both a stack cap and a duration. Terror does nothing, until. . .
Darkness - OFF-GCD skill that consumes stacks of Terror on nearby targets to grant the Dark Knight a shield/damage reduction/etc.
I think this would be a cool way to let the Drk mitigate the big hits that we've come to expect in tanking raids in this game (Death Sentence/Raven's Beak/etc), while keeping it different from just a boring Cooldown to pop. The stacking with a duration and a stack limit means gives the move this pro and con that I find interesting.
1. You'd have to pay attention to your stack number and time left so you can't just set it and forget it.
2. However because it does cap out, after you reach the cap as long as you hit it with a combo every other rotation or so, you can use the downtime to cast other moves/taunt adds/use a different combo/etc without feeling penalized for not using your basic 1, 2, 3 over and over.