Legend is a story not something in the past alone. Plus the pokemon are legends. And the stoeybitself could be a legend and more info on the war from 3000 years ago which would fill your definition which is t 100% correct

I don't know what game you played but Arceus was not difficult at all. The only thing that made it difficult were the final bosses and the pokemon battle.was mostly because no one was ready for it and lasting as long as it did. If you want to worry about something worry about the size. And Arceus was far from a master piece.