Hum. It's the sneak past death knight that has me worried. Where is that key you spoke of - is it near a teleporter? My character has perception 10 but i honestly don't remember seeing any thing about a key (or place to dig). I do remember a locked switch that didn't work that I couldn't figure out so maybe the key is related to that ?
In the very west of the cave in Hiberheim there is a fire area that you can only enter with a key which you need to activate a lever. You can find the key just a few meters on the right in a small basin with blue mushrooms in it. You have to dig it up.
For the sneaking in the mine. Just seperate one char from your group who can cast invisibility. Scout with him the area until you find the second portal. Go back to the group. Leave the mine through the entrance. Teleport yourself to the second portal in the mine. From there it's easy to get to the temple door if you choose the southern passage.
