This chest grants reputations for Athena's Fortune to those who cash it in. This chest is cashed in to the Mysterious Stranger in the Tavern at any outpost. Just like most chests, the Chest of Legends can also be found in the ashen form, increasing the experience and gold gained from selling it.
Normal and Ashen Athena Voyages for the Chest of Legends are obtained at the Athenas Fortune (the pirate legend hideout) from the Pirate Lord. Note that you can buy the Ashen Athena's Voyages from any outpost due to them all teleporting you to the same Athenas Fortune.
While the Ashen Chest of Legends is worth varying amounts of gold it reputedly gives double the amount of XP in comparison to the regular Chest of Legends.