The Lake of the Lost (aka Mordavia Lake) is a deceptively tranquil lake on the western edge of the Land of Mordavia. It can be reached by travelling four screens south from the Castle gate, then one screen west, and then another screen south. Entering the Lake screen cancels the action of being followed by a monster, both during the day and at night.
The Rusalka lives in the lake. She would appear as soon as the Hero approaches water, by day.