In modern speak, Empire was the point where you the viewer - and The Rebels - got served with the "this shit just got Real" card
It could have even worse; if Vader's mission plan had been followed they all might have been in the bag. Or worse, if Vader wasn't in charge, but instead say some Clone-War era veteran who followed pure military procedure and took the shield down, then bombarded the place to molten snow and slag. But its the mark of someone in the production of Rebels, that the writers must have had a minor revolt - the good ones anyways. If you want to be taken halfway serious, then you must be more than just an annoyance. And there must be a serious threat. For good or ill, Darth Vader is the apex of that. What still remains shocking is the number of fanboys who still dont get it. Who think that somehow Asoka is going to BEAT Vader, or make him all squishy nostalgic for his life. Perhaps they fucking slept through Ep III??

Vader is a Monster. Obi-wan wasn't just playing word games. Anakin IS dead. There is only Vader. You think Anakin would have just butchered those jedi younglings for giggles and not shed a tear? Aside from the canon, Vader is even MORE powerful now than when he was just a man. Focussed. Controlled. Channeling the Dark Side like crackling Black Lightning. He has mastered what Dooku said he possessed: FURY. If anyone's mind is going to be fucked, it's going to be Asoka's, since Vader knows who she is and all the other little things he could use to twist and torment her. Remember, this is the guy who tortured Solo to a screaming hulk just, because

He knew Skywalker would sense their pain and anguish, and come running. Delicious....
