[Radek]

Another possibility of disappearing important items is thieving creatures. All ratlings, all higher level trolls, all highland rogues, and perhaps some other. Note that a thieving creature can steal everything that isn't equipped: normal items, keys, quest items, money... rrrrrraugh!
If you've let some of these nasties to go away after having to "communicate" with it, then the shroud could go away as well.
Always kill thieving creatures and always check what they have dropped. You can find precious items sometimes - your items. Note that a thieving creature always drops a stolen item after being killed - except stolen money, which aren't dropped. Rrrrraugh!

In your current situation, you have two possibilities:
1. Kill Erathsmedor - the door to the Mavin Sword will open.
2. If you do not feel to be strong enough then you can vanish through the pass on your way to Brimloch Roon and continue the game. You will not need the sword until you reach the Pyramid. You can get stronger first and then return to Erathsmedor, kill it, get the Mavin Sword, and finish the game.