Ha! Had to get that reference in there for the show, "How I Met Your Mother" just because I can.
But I have to correct my title. L isn't for Legendary, but it is for Legends.
In fantasy world building, you have your gods and goddesses, your heroes and your sinister beings hell bent on ruling the world. Magicians and dragons, Elves and Dwarves, Knights, Lords and Ladies... (Ooh, more 'L's'!)
But you also have legends.
These can be stories passed down from generation to generation.
"History became legend. Legend became myth. And for two and a half thousand years, the ring passed out of all knowledge." ~ Lord of the Rings opening narration.
Stories become history. History becomes legend. This is absolutely true. This can be events that shaped the world at the time they happened. They can be events that shaped the world later, after they happened. They can be people.
Think about some of the fantasy worlds out there that you know of. I'll give you a couple examples of books that stand out in my mind.
In "The Dragon Prince" series by Melanie Rawn, stories and history turned into traditions as well. Every few years there was a dragon hunt. Rohan was told by his father to hunt the dragons to keep the population down to protect his people.
Rohan, however, discovered the truth of why the dragons were hunted and killed. When the dragons hatched from their eggs, they spewed fire to dry out their leathery wings. But when the dragon-fire hit the eggs, they turned into gold. They weren't killing the dragons for population control, they waited until the dragons hatched to take the gold. And so dragon's gold became a legend.
In "The Belgariad" series by David Eddings, Belgarath the Sorcerer and his daughter, Polgara were legends. Stories were told of their deeds and influences on the world. Polgara herself even admitted to involving herself in politics and wars. Even when she wasn't supposed to. But things they had done were in the history scrolls and became legendary.
So what do you have in your world that helps shape and reshape your world? Who or what in your world is considered legendary? You have the foundations of how to build legends, now go forth and be epic!