I've read stories from the Imperial Library, but I don't know if they're true, about the ancient might of Ra Gada. They, apparently, destroyed an entire race with '5 Giant Swords' and literally deminished the entire race to a smaller size with magic. Apparently they are the only race with cannons and have heroes that have formed pure swords ("soul swords"?) from magic (or some other life force) and can slice atoms and do all sorts of crazy stuff.

Which of this is actually true in TES lore?

Thanks.

Chances of them having cannons is almost nil. They are mentioned once in a Joke book found in Daggerfall. (I think thats the only mention)

Everything else is basically "true." Then again it could just be stories that some kooky Redguards thought up after they messed up Yokuda. There arent any remnants of the Left-Handed elves and the only information we have on them is from Yoku sources saying they killed them off.

Shehai(The spirit sword) and Pankratosword(their atom splitting sword style) are both lost arts (especially the later)

The cannons part has more truth to it then a joke book, there were supposedly cannons in the game Redguard.

I dont remember seeing them. They might have been in the background... but thats a big maybe... Its been a long while since i've played it though

Cannons are used in skyrim when you assault Japhet's folly for the EEC. Its not too suprising that the redguards probably have them too.