» Mon Jan 28, 2013 5:39 am
They're refugees from a continent called Yokuda that was mostly exploded as a result of some kind of nuclear arms race (except with crazy sword techniques instead of uranium piles). A few of them washed up on Hammerfell, murdered the local orcs and goblins until they ran away screaming, and they settled down and built some cities and got a reputation as kickass warriors and sailors. The first wave of settlers was called 'Ra Gada', which got corrupted into their common tamrielic name. They've got their own pantheon of gods that don't really correspond to any commonly know Aedra or Daedra and have names like Hoonding and Tuwhacca, though I think by the time of the regular games most Redguards worship the Nine.
This is mostly off the top of my head, backed up by uesp.net. The bit about sword-saints causing nuclear war is from a possibly-non-canonical kirkbride vignette where Cyrus, protagonist of Redguard, meets Lord Vivec, but I can't find a link for that one right now.