The Legends Are True

One of the pervasive tropes of high fantasy is the almost-forgotten legend that turns out to be true. Coming from a cultural background that prizes oral tradition and group histories, this seems completely reasonable to me. After all, many cultures have narratives about historical beliefs and events stretching back for thousands of years. That said, these having an ancient legend as a plot device can cause a number of plot holes. Here's some questions to help you place the legend in the context of your plot so that it's a plot point instead of a plot pitfall.

Have any inaccuracies cropped up in the story? The longer and more widely information has been circulating, the more likely it is that details have been lost, changed, or exaggerated.
Is this written down, an oral history, or some combination of the two?
Who knows the story? It could be common knowledge, a culturally specific piece of information, or something that is primarily known within a certain profession.
If it's common knowledge, why are people only acting on it now? There are all kinds of reasons something that was seen as an important part of the past becomes critical to the present.
If it's restricted knowledge, why? Maybe  the story is only circulated in a particular region, or among members of a particular cultural or linguistic group. Maybe a certain group has given up their belief in the story. Maybe it's a closely guarded secret.

If you flesh out how this legend is incorporated into your society as a whole, you have an excuse to sneak in some worldbuilding and make your setting seem more real. The appearance of the legend as a plot point will also seem more believable. Both of these things will help your audience suspend disbelief and enjoy your story.