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
I am level 30 and have completely finished mirkholm keep solo as pure necromancer. i find that i must now go elsewhere to level up further to be able to continue any further in Grizzleheim.
Grizzleheim is a subsequent world. Once you get to level 20 and return to Grizzleheim, you will get a new set of quests every few levels. Areas are in this order:
Savarstaad Pass-20 Vigrid Roughland-30 Mirkholm Keep-35 :-) Ravenscar-40 Nidavelir-45 (Although you can enter at any time.) :-P