Grimvale

Grimvale is a small island located north-east of Stonehome and Edron. It can be reached by taking the Small Boat with Cornell, located here.

According to Lionet: "the moon will be full around the 12th-14th each month (the 13th of each month being the prime time for the events on Grimvale). We introduced these "moon phases" to be a constant phenomenon around the 13th of each month during the whole year." (source)

This means depending on the time of the month you are visiting Grimvale, you either encounter fairly harmless creatures such as Badgers and Bandits, or a lot stronger creatures such as Werebadgers, Werebear and Wereboars.