It doesn't really seem all that powerful as a GCD. It's usage is a very minor dps increase because of the way our kits work, and because it delays our regular combo it might not even be worth it in some situations depending on if you need MP for burst windows.

As it stands, bloodspiller feels like a replacement for scourge that does less damage per button press but happens more frequently.

We also lost Low Blows and Reprisals which added to the speed/responsive feel of playing DRK, and the increased cost of dark passenger makes it not spending mana on during single target, making us lose another oGCD.

The class will feel extremely boring.

But as an oGCD, you could do some interesting things. It could be banked for burst windows without much detriment.

Damage can always be tuned so that this wouldn't be op but I've always enjoyed the speed at which dark knight was played, with all it's oGCDs. The immense slowdown just increases the similarity we have to 3.0's paladin.