NOTE TO Franz: All of this information here in the 'Main Quest' lore pages is not known by the NPCs unless they are specifically mentioned to know about a piece of information.
Focusing on the town of @Stillwater first and setting up a base of operations there can be a big help for exploring the Black Mountain Mines.
The first two levels of the Black Mountain are full of evil creatures. How you get through them depends on your playing style. Don’t be afraid to avoid confrontation and/or come back later if you can’t seem to get past. @Oregolems, Seething Masses and Granite-Rats can be particularly deadly for low level characters.
At the end of 2nd level you will start to find dead @Molochean Hand members. The tunnel here goes back to an unreachable part of level 1. Ahead of you is two doors. The closest door leads down to you eventual destination, while the other door leads to more rooms you can explore (with more evil creatures and @Molochean Hand members alive and dead) These levels begin to be trapped very heavily and can decimate you party if you are not careful.
After some exploring you will come to a long trap infested tunnel. At the end of this you will find Gudmund Ore Bender and the pillar of truth.
This will tell you that the @Black Mountain Clan have been banished to the @Isle of Despair.