Yes indeedy, this was a bit of a corker wasn't it?
*SPOILERS!!!!*
It does make perfect sense for Dooku to commit such resources to destroy Ventress and the Nightsisters, after all remember they've already made two attempts on his life. First when Ventress and a team of Nightsisters attempted to assassinate him, and then again with their double agent and his new apprentice Savage Opress, so some form of retribution was always on the cards, just a matter of when.
Like you say it was definitely dark, you wonder how the Nightsisters are going to take on the full mechanised might of a full droid army (with General Grievous at it's head), with starfighters bombers and the like, but they actually quite convincingly hold their own for a time, but I always thought they'd have to lose in the end, but when the Voodoo element came in I thought, hang on, they might just pull this out of the bag, but then in some unusually logical and realistic writing for the Clone Wars, even though Dooku's life was under immediate threat he reasoned that he couldn't allow Ventress and the Nightsisters to survive and continue plotting against him. So this plot element which could have drawn the conflict to a close in a bit of an unsatisfying way was raised and then rejected, resulting in no easy way out, this was going to be definitively resolved one way or another right now. Which was a refreshingly bold move. So Dooku stays the course through the pain being inflicted upon him, and finally the droid army are victorious, reaching the Nightsister leader Mother Talzin and breaking the Voodoo spell she held over Dooku.
Strong and dark storytelling here, taking in black magic, Zombies and Voodoo, not to mention an excellent duel between Dooku's two rival apprentices Grievous and Ventress, even this is beautifully handled, with Ventress just about getting the better of Grievous, and then the treacherous cyborg droid leader doing what he does best by going back on his word and commanding his army to take out Ventress when their duel turns out bad for him, playing perfectly into his movie counterpart's cowardly nature.
So this episode ends with the Nightsisters defeated, Ventress wounded and all alone once more, where could this go now? I'm feeling that the return of Savage Opress and the proper Clone Wars debut of Darth Maul could be just over the horizon.
It is also interesting to note, that once again Clone Wars seems to be at its best when the principal hero characters are not involved, what does that say?