During the course of a Season, there are some rewards which are exclusive to only Pirate Legends. While becoming a Pirate Legend during a Season will allow the current Legend rewards to be granted to your account, this does not back date and grant Pirate Legend rewards from a previous season.
For example, if you turn Pirate Legend in Season Two, you will not be granted the Season One Pirate Legend rewards such as the Pirate Legendary Curse.
