According to a widespread belief, related already by Olaus Magnus in 1555, witches meet there each Maundy Thursday.
Blockula is originally the same place as the island Blå Jungfrun, which was in old days called Blåkulla, and since medieval days rumored to be a place were the witches gathered.
The Blå Jungfrun is a small island just outside the coast, and is classified as a National Park.