This is for finding and selling one of each six (6) different new treasure items, which you can only get from the new 'Athena’s Run of Thieves’ Haven' voyage, available to Pirate Legends, for 50 doubloons from Duke in any outpost. If you are not PL yet, you can invite one into your crew to buy the voyage for you. The items can only be sold to the Mysterious Stranger in any outpost.
This is a long voyage with 5 chapters, and it will have you return to Thieves' Haven 3 times, digging up about 17 items total. You'll most likely not find all the Legendarh Treasures in one voyage, since some of the items will be regular Mermaid Gems.
However, since there's an commendation (not achievement) for doing it 5 times, you'll most likely get all six items if you're going for that too.
The items are:
* Chalice of Ancient Fortune - an egg-shaped purple trinket
* Crate of Legendary Voyages - a merchant crate, glows blue
* Skull of Ancient Fortune - a foul skull, with slight blue glow
* Gilded Relic of Ancient Fortune - a golden cup with blue/purple inside
* Villainous Skull of Ancient Fortune - a villainous skull, glows blue
* Keg of Ancient Black Powder - a stronghold gunpowder barrel, glows blue
Here's a picture of all items together, from the SoT website:
Added tip provided by
KawiNinjaRider7: on the islands that are not Thieves Haven, digging up loot can spawn a Shadow of Fate skeleton (which can only be killed by first shining the corresponding lantern light color on them), which have chance of dropping a Legendary skull. It's worth bringing those lantern colors back from the Ferry just in case. I've seen them in at least red (killed by fire) and green (killed by skeleton).