Fraywrack (5E)

From publisher blurb: When a flight of harpies, beset by a relentless demonic menace from beneath the dark seas, used their captivating song to lure a ship to ruin, they sought allies instead of food. Thus, was born Fraywrack, a war camp created by the strange alliance of man and monster against a far greater threat. Now, hidden by the broken remains of their ship, the wreck’s survivors and their later recruits train endlessly for a fight they fear they have little hope of winning. Still, they persevere, welcoming adventurers who may wish to learn some lessons of their own or, just maybe, lend their own might to the struggle.