Berserk Potion Quest/Spoiler

You need permission from Hjaern to travel Helheim

Get to Helheim, go up in the stairs to the south-east, go down the hole, where you will find mummies and ghouls, follow to the extreme south and go downstairs, then go west until you find another staircase, go down, where you will find demon skeleton and some bone beast, now follow the line walking to the north to blue circle, open the skeleton to find u reward.