The vault must be hidden somewhere around here, I just know it. See if you can manage to get any of these strange aliens to speak with you, I tried and they just screeched at me. If not, then see if any of the fallen adventurers around here had a clue on them before they became undead. I'll just be over here reading some of these old books I found and seeing if they have any clues in them.
"Ancient civilization… blah blah… Citadel… Shadowscythe… Infected… Stranger… Blah blah blah… Betrayed… Doom…" Hmm. Huh? Oh sorry, was I mumbling? I tend to do that when I'm researching, but all the books here describe a darkness that infects people, and an ancient civilization with massive ships that fly through space. There was also an entry on a stranger in a cloak that caused some downfall after a betrayal. Crazy right? I don't believe any of it myself. Wait - you found the clue already? Great! What is it?