The Dissidia Board on GFAQs seems to be certain that it is indeed fake. I didn't look much into it, but there's several topics going "What are those fake Kain scans?" and "Who made them?"
Furthermore, Chaotic Cosmos, which is pretty much always up to date on Dissidia info, didn't mention anything of this (though they did mention the "Lightning in UNiversal Tuning!" claim which turned out to be photoshopped.)
Edit: Apparently, one of the ways its confirmed to be a fake is that if you look at Kain's shot, namely his hair, its IDENTICAL to a section of Dissidia Cecil's Hair, an unexpected section, but still that. Furthermore, you'll note the scan is incredibly blurry and pics are really small, and to be frank, someone noted that something like this would be rather wide spread by now, not kept to one set of blurry pics.
Compare those pics to other Dissidia scans we got, for example.
Doesn't 100% confirm its fake, but it does make the info skeptical until we get something a lot clearer.
