Sonderland is a country west of Skandia; not much is known about it. It may be based off either Iceland or Greenland.
We don't know anything about it's relations with Skandia, but as there are trade fleets which sail from Hallasholm to Sonderland we can assume they are trade partners or even allies (as Iceland and Greenland were Viking colonies, Sonderland could also be a colony of Skandia).
Sonderland lies across a strait from Skandia. The strait is filled with countless small islands. Sonderland also forms a strait with Gallica and borders the Stormwhite and Narrow Seas.