Noun which may take the genitive case particle 'no', Noun
once-in-a-lifetime (opportunity); golden (opportunity); one-in-a-million (chance)
Yojijukugo (four character compound)
いよいよ千載一遇のチャンスがめぐってきた。At last, a chance in a million arrived.
Also written 千載一隅【せんざいいちぐう】