Many legends surround the fresh water lake and the nun from the society of the Ice Serpent who drowned herself to close a gate at the bottom of the lake. Supposedly, the gate leads to another world in a far-off cluster of stars where strange gods and ancient evils dwell. The nun was transformed into a banshee that haunts the lake shore when there is danger of the gate being opened. Some halflings fearfully whisper that the nun has been sometimes seen at night in the past couple of decades. At the same time, there have been reports of strange foot prints found at the shore. Foot prints some crazed magic-users believe to belong to Deep Ones. But of course that's typical villager talk.
Note: The Lake of the Spooky Nun can serve as a starting point to introduce monsters, artifacts and rituals from Carcossa into the campaign.