I did all the available quests in all the areas and the last quest I finished was in Frostholm but to continue looking for Ivar Anderson I need to get into Mirkholm Keep but I cannot find any source of any game character that will send me there.
Which NPC do I talk to to get sent to Mirkholm Keep?
Okay. See, you have to finish Savarstaad Pass and Vigrid Roughland by doing all main and side quests there. Once you finish those areas, wait until Merle Ambrose call you to visit his office and go speak with Bjorn Ironclaws go then Mirkholm Keep is unlocked! :-D