I've freed the white witch; but lady by the lake quest did not close;
After your arrival in hilberheim there you need to dig for a key, which can unlock a poisoned area there is the dying "lady by the lake" you need to talk to. I also missed this area totally.
I found a mine but death knights chased me out (can't harm them); did not find any t. ore in it; did find a book that said i needed some kind of spell to kill the dead knight (no clue where the spell is or how to recognize it; btw the book is too close to the dead knight)
You need a pickaxe to collect tenebrium ore, but you can also find some lying around somewhere in the mine. To progress you need to sneak past the death knights until you reach the temple mentioned in your quest log. There you can find a spell scroll to make death knight vulnerable.
Found the phantom forest but the imp told me i needed special weapons to proceed and i don't have them and no clue where to get them.
Collect some tenebrium ore for brandon in silverglen (from mine), then you receive the ability to handle tenebrium. You can then forge tenebrium ore into tenebrium bars and enchant you weapons with this. Alternativly you can find some tenebrium weapons in the mine.
Tip: Store all tenebrium stuff execept 1 ore in a random crate and then give it to brandon, this way you have plenty of tenebrium to play with.