Nidingen is a Swedish Kattegat sea area skerry with a lighthouse station. It is located in Onsala parish in Kungsbacka Municipality in the province (landskap) of Halland. The island has with its surrounding reefs always posed a danger to shipping. The three lighthouses on the island, one of which is active, are today protected as heritage buildings. The island is also a nature reserve with a birding station and hostel, famous for large populations of different bird species. The province of Halland and Nidingen was ruled by Denmark until the Second Treaty of Brömsebro 1645.
https://en.wikipedia.org/wiki/Nidingen