An article-less noun can mean a general or original purpose of the word. Is that why legend has no article? Doesn't it mean any specific legend, but general legend?
ex)Have you heard of the story of King Arthur? It is an old British legend about how Arthur became a great king against all odds. According to legend, Arthur was the first son of King Uther Pendragon....
Are these the similar examples?
1)I go to a school => go to an unspecific school(physical meaning)
2)I go to the school => go to a specific school(physical meaning)
3)I go to school => No physical location implied, just the action of going to school for learning, the original purpose of school is realized
1)George Bush left the office =>He left the building of his office to leave for the day.
2)George Bush left office => He resigned the presidency. No physical meaning, just the action of resigning one's post.
For countable nouns' generalization, you normally use plural forms like
ex)Dogs naturally bark, Doctors are the people who help others, Human beings should love each other.
But for uncountable nouns' generalization, you normally use singluar forms like
ex)Water is used to drink, Pizza is tasty to eat, Love is the supreme thing,
According to dictionary, "legend" can be either singular or plural, so whenever I see this kind of words, I'm confused about whether to use "legends" or "legend" for generalization, so I have temporarily decided there's no fixed rule for this, there's much more idiomatics usages.
So I feel I'd rather just memorize those usages than try to set up a rule for this. What do you think?
It's pretty safe for you to use 'legend' as a countable noun. In the original, "According to legend, Arthur was the first son of King Uther Pendragon...." it would be acceptable to use 'a legend' or 'legends'.
Simply note, for your own interest, when 'legend' is used uncountably. You'll probably find that it is used in this way in a small number of almost fixed expressions -According to legend, Legend tells us (that), Legend has it (that), the heroes/witches/knights errant/dragons of legend, ...