noun which may take the genitive case particle 'no'
legendary; fabled; in legend
Used before a noun with の to describe something that exists or is described in legends, not necessarily implying it is real.
I read a book about legendary creatures.
The story is based on a legendary hero.
Compound of 伝説 (legend) + 上 (above, on), literally 'on legend' or 'in legend'. The exact historical derivation is uncertain, but the suffix 上 is a common way to form attributive expressions meaning 'in terms of' or 'from the standpoint of'.