Lendário vs. legendário
In Portuguese, lendário and legendário share the same etymological roots and are often listed as synonyms in dictionaries. However, there is a distinct difference in usage frequency and nuance. Lendário is the standard, modern term for something mythical or famous, while legendário is rarer, often archaic, or refers specifically to a collection of written legends.
Lendário
B2This is the most common word used to describe something pertaining to a legend, a myth, or something fictitious. It is also the correct term to describe a celebrated, famous person who is considered a legend in their field.
O Rei Arthur é um herói lendário da Grã-Bretanha.
(King Arthur is a legendary hero of Great Britain.)
Aquele cantor de rock teve uma performance lendária ontem à noite.
(That rock singer had a legendary performance last night.)
Não sabemos se o animal é real ou lendário.
(We do not know if the animal is real or legendary.)
Ela herdou uma espada lendária de seus antepassados.
(She inherited a legendary sword from her ancestors.)
Legendário
C2While this word can serve as a synonym for lendário in formal comparisons, it is much less common in daily speech. It is more specifically used as a noun meaning a book or collection of legends (especially regarding lives of saints), or as an adjective referring to inscriptions and subtitles.
O padre consultou o velho legendário para ler sobre São Francisco.
(The priest consulted the old legendary to read about Saint Francis.)
Este manuscrito pertence ao ciclo legendário medieval.
(This manuscript belongs to the medieval legendary cycle.)
O estudo legendário foca na origem escrita dos mitos.
(The legendary study focuses on the written origin of myths.)
É uma figura do passado legendário da nação.
(It is a figure from the nation's legendary past.)
Summary
To sound natural in Portuguese, you should almost always use lendário when describing mythical creatures, epic stories, or famous people like athletes and musicians. You should generally reserve legendário for academic or literary contexts that refer to actual collections of written legends or hagiographies.