1. Okay so I learned I need tenebrium weapons to affect these void demons from the imp fellow , However I do not have the skill , Did I miss something ? as I'm honestly not sure where I acquire this skill

2. the Goblin Mines , So there's a spell that will make these Death Knights killable , is it learned inside the mines or some where else? as I lack any sneaking so I doubt I can just sneak by them.