Ship of Souls
Uncommon23.7%ObjectiveOptionalAnywhere
Set sail in the Order of Souls' name and gathering skulls will boost your fame.
You must first be level 15 for the faction, and then buy the emissary flag for 20,000 gold from their representative at any outpost. Then (in the same room/area as the representative) vote to be emissary on the nearby table.
This will unlock if you're in a crew and you only vote - i.e. even if you're under lvl 15 and someone else buys the flag.
Conversely - you can also do this solo; just buy the flag and vote solo to raise it.
NB being an emissary increases XP gains significantly - helping with other achievement grinds.