Thanks for the hint Zombie Yeek, but I know the dungeon spoiler. Unfortunately I don´t see how it answers my question (an answer to which might be too spoily for this forum, as I realize).
Pay attention to the numeric values in the spoiler. To make the matter more clear, the method you are asking of is basically the same as it was before the creation of the Void (which is missing from the spoiler).
I found a level in the void where I couldn´t go down anymore, but when trying, it told me to do something. I figured I had to kill someone here, but didnt find anything special. I left and returned for a few times, but nothing. Eventually I ran out of healing resources and died.
Have I missed some ingame instruction/hint since the message of Lady Galadriel or is this quest not supposed to be done without spoilers ?
DG: this, combined with droma.'s earlier complaints, should probably be seen as support for my repeated suggestion that the Nether Realm be done away with...