Unlocked by selecting the Athena's Fortune Emissary Flag, this is unlocked by reaching level 50 in 3 of the other factions and buying an Emissary Flag from the Pirate Lord in his tavern for 20,000 gold.
Then in the overworld tavern, voting to raise it next to the Mysterious Stranger.
This can also be unlocked by having anyone else on your crew purchasing the flag and voting to select it.