This is a 5th edition adventure for level 5 characters by Goodman Games.
From the publisher:
Fifth Edition Fantasy is here! This adventure module is fully compatible with the fifth edition of the world’s first fantasy RPG, and is ready to play in your home campaign!
Mistcutter Isle has always had a dark history. For years the isolated isle served as a haunt for pirates and smugglers looking to hide their ill-gotten gains. But before that, legend has that the Isle was home to a race of savage, sea-dwelling creatures that enslaved other races.
Somewhere on the Isle, it is said, is their hidden temple. Such tales were not always believed, but recently sailors have seen unusual purple lights in the sky above the Isle. Something is happening, and those with long memories fear that evil is afoot. Do the adventurers dare investigate the secrets of Mistcutter Isle?