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I'm compiling in-world legends for my built world, and would like them to feel like established legends from our own human experience.

What should I keep in mind while writing these legends? What elements, styles of prose, turns of phrases, structure, length, and so on are more likely to be found in legends than in a more routine work of fiction? What makes a legend sound like a legend?

I think of legends and mythology (I took a college elective on it) as being about black-and-white extremes, like writing for children too young to process nuance, too young to appreciate flawed heroes or sympathetic villains. Hercules is the good guy. Heaven and Hell, Mount Olympus or the Underworld, are polar opposites, it is either 100% good or 100% bad. God is great, Satan is completely evil.

Other than that, the story structure is much the same. Joseph Campbell's The Hero's Journey applies; he derived it from studying myths and legends. It fits neatly into the Three Act Structure, but contains more detail.

Myths and legends are used to teach idealistic life lessons (now and always have been), in particular to young children, in an entertaining and memorable way. Hence the demand for simplicity and extremes in motive; and a lack of nuance, a streamlined plot structure without side-trips or dithering. By "idealistic" I mean the hero is brave, intrepid, and puts himself above others. If he acts selfishly, he is punished until he makes amends. But he doesn't give up! Evil gets vanquished. The Wizard helps the hero because the hero is righteous, or has a good heart. Heroes may be clever and outsmart the monster or villain, but only the villain's actually cheat or break their word.

Now I suspect you may find exceptions to that in actual myths or legends. Aladdin may win the princess by employing clever lies; but in the end Aladdin has a good heart and a true love so this is forgivable; he is not lying to trick the princess into a romp in the hay.

But the principles hold. Think of the legend as an idealistic story for prepubescent children, in which the hero has (or develops) several traits of the best mankind has to offer: In particular bravery and altruism, self-sacrifice to do what is right or to punish what is wrong. They protect and defend good women and children. The gods are on their side.

Make sure the villain and obstacles have the worst traits of man or nature. The villain is a duplicitous liar, cheat and fraudster. A trickster. Perhaps they will kill without compunction, mercy or regret. The storms are ruthlessly violent, the desert is blistering hot. The obstacles are literally lethal monsters that must be killed for the greater good.

Legends are defined by the cultures who created them

Legends reflect the values of their culture. Sacred hospitality and the inevitability of fate were popular themes for the Greeks. The Norse Eddas focus on personal sacrifice for power and knowledge several times. Russian fairy tales celebrate kind fools. Some cultures revere tricksters, other demonize them.

Legends often have poetic forms

Legends generally begin life as oral traditions. Poetic forms make it easier to remember long stories, so they're very common among the large epics. But it depends on the type of story being told. An epic tale told by a master storyteller will be poetic - a fairy tale told by a parent won't be. (Although there can be a certain rhythm to children's stories as well).

The exact forms will be different from culture to culture, and will be shaped by the nature of the language.

Apart from Campbell's "The Hero's Journey", another source of archetypal knowledge is folktales. You could take a look at the work of Vladimir Propp and his analysis of Russian folktales.

Both Campbell's and Propp's works describe a structure common to many stories of each category: the repetition of three, leaving the normal world, the appearance of a guide or mentor, being sent on a quest by the king, etc.

Many myths and legends (most?) have a religious source, so figuring out an "old almost forgotten" religion (e.g. ancient Greece/Rome), and Gods and Heroes from that religion might give your legends a backbone.

Folktales usually don't have that kind of source.

Legends and myths are also, sometimes, used to explain natural phenomenon:

Persephone was forced to live in the underworld for half a year so her mother Demeter, the goddess of agriculture, harvest etc, mourned and there was winter, and then when Persephone returned Demeter was happy and there was spring, summer, harvest...

Both myths and folktales belong in the oral tradition, so they usually have more psychological and symbolical depth than they seem at first glance. Many people (and their minds) have bent, twisted and adjusted the story to give it that depth.