A long time ago, there was a well known boss known as Jotun, and his brothers Ullik and Grettir. The area you had to go to to access his painfully long dungeon was Nidavellir, a "secret" place that could be accessed by anyone of any level. But now that I come back to Grizzleheim and try entering the large door that leads to Nidavellir, I am unable to go through it. It says in white text "To Nidavellir" but simply nothing happens. Even the lantern next to the door is lit yellow, indicating that this area is accessable. So I was wondering if the area was removed due to a nerf, or do you have to get to the point in the story to get there? I would really like to know, these bosses and dungeons were a big part of the days when I first started playing.
You must be at least level 40 and have completed Mirkholm before you will be sent to Nidavellir. If you're past level 40 and did finish Mirkholm Keep, go check if the King of Grizzleheim has a new quest for you (you may have missed his summons). He'll tell you Jotun wants to challenge you.
Go into the central dungeon and you will find that Jotun is still there in all his itchy, scratchy glory! But be warned: If you go to Jotun without completing the side dungeons first, you'll have to fight Jotun along with his two evil brothers.
Fighting Jotun in Nidaviller is required to progress to Ravenscar, and then to Wintertusk. You must be level 40 and have completed Mirkholm keep. If you have completed the prequests you will be told to go see the bear king in Northguard. Jotun is more difficult by far than anything you see in Mooshu or earlier, and I think a bit tougher even then the battle with Malistaire at the end of Dragonspyre. Teamup is your friend.