You'd think Ben or Yoda would have mentioned it, seeing as it was a pretty common technique wielded by darksiders in the PT era and before. Both of them faced it during the clone war era, Obi-Wan facing Dooku's lightning on at least two occasions, and Yoda facing both Dooku's lightning, and later Palpatine's lightning when he fought him in the senate building. Since it was pretty likely that Luke, in going to confront Darth Vader, might end up confronting Palpatine as well, it would make sense for them to warn him specifically about this power and its possible use against him. But Luke seemed to be caught completely flatfooted. "If you will not be turned..." *Palps raises hands slowly into force-lightning attack position* *Luke stares blankly* "...then you will be destroyed!" *Palps starts zapping.* Luke gives no indication of having anticipated it, despite Palps telegraphing his attack such that anyone, never mind a Jedi, should see it coming.