Every 10 years, during the longest night, an island supporting a seaweed covered ruin rises from the water. Terrible monsters emerge every night to terrorize the countryside before the island sinks back beneath the waves a week later.
This adventure can be can be easily customized to a party of any size or level, or run as a level 6 adventure for 5 players. It can work as a small side adventure in an existing campaign, as a standalone one-shot, or the start of a new adventure.