This is me trying to track down a vague rumor from childhood, so be warned. When I was in elementary school, somewhere in the late 1980s to the early 1990s, I remember being told that Emperor Palpatine, in expanded materials, had a lightsaber with a curved blade. Specifically, while it was nominally a normal lightsaber, he used the force to curve it to show off his power or for some tactical advantage like being able to to circumvent an opponent's guard (the normal reason for a curved sword, concentrating the force of a blow, allowing more powerful slashing blows with a lighter weight, does not really apply to a lightsaber). They swore that it was in the books — I want to say that it was somehow tied into an idiot savant weapon designer who mumbled new designs in his sleep, but was functionally retarded otherwise.
I've found a few references with people claiming that this was supposed to have been introduced into Episode 2. And there are other people asking after canonical references. Obviously, this is something that has heavily weighed on people's minds, if only because people like to think of ridiculous lightsaber variations.
The closest match I've found so far is the Darksaber, which has a curved edge (somehow), but that was introduced way later than the timeframe I remember hearing about this. It's also possible that they might have been thinking about an Echani blade or the like, but the description of it being curved by the means of the force doesn't quite match there. It's also possible they were just making it up, I guess. It was elementary school and goodness know we all made some pretty huge fibs back then.