[English] The single naga
'Save all golden nagas in the grotto and''return to the single naga at the desert''caves.'
Here "the single naga" sounds a bit awkward. I think first of all, because "single" means "non-specific, just one", while the phrase suggests "exactly this one". Note, that "single" also means "not married". Maybe simply leave "the naga" - and it'll be perfectly clear that it's "the naga hero had spoken to earlier". Or maybe "the lonely naga" which would stress loneliness, desire for fellowship.
Note, that saying "I saw a single man standing on the hilltop" sounds nice. In other words, I may be wrong about this (while I have C2 in English, still, it's a foreign language to me), so if you are able to consult someone whose native language is English, that'd be cool.