Solstheim is an island to the north-west of Vvardenfell and east of Skyrim. It is inhabited by a number of Nordic tribes such as the Skaal and by Dunmer refugees who fled Morrowind. Solstheim is also the primary setting of The Elder Scrolls V: Dragonborn. The Dragonborn can reach that location after reading the Cultists' Orders.
The geography of the island bears similar traits to both Vvardenfell and Skyrim. The north is dominated by the Arctic climate similar to that found in Winterhold, while the south features the jagged landscape reminiscent of the Dunmer.
Following the eruption of Red Mountain in 4E 5, thousands of Dunmer were forced to flee Morrowind. Many crossed over the border into Skyrim seeking refuge, and whilst they were accommodated, the Nords graciously offered them the island of Solstheim] as their new home. The High King of Skyrim gave them this territory formally in 4E 16.