Svartalfheim (the Realm of the Dark Elves), is the mostly underground home of the Dark Elves, better known as Dwarfs. It is the seventh in the spectrum of the nine worlds in Norse Mythology and borders to Jotunheim and Helheim.
Whoever gets invited to the Dwarfs domain can easily be 'taken by the mountain' and kept as prisoners for the rest of their lives.
Svartalfheim is a dark world of deep rock and underground tunnels. The Dwarfs have built great castles and homes in the mountains here, and their prowess as blacksmiths and craftsmen is widely renowned.