From Whence They CameG 30

A straightforward cumulative achievement, just kill 250 phantoms (ghosts) while at sea forts. Forts are hard to miss, they’re the heavily-armed square fortresses that give off a green glow when they’re populated with enemies.

Note that while you can clear out forts with a few crew mates, only your kills count towards this achievement. Any phantoms your crew kill, count towards their progress and not yours. But forts refresh themselves quickly and are a reliable way to make some gold so grinding out 250 kills won’t take too long even if you’re only getting a share of the kills.