This is supposed to be a bit of a puzzle. I can give the answer outright if you really want, but for now I'll just give a pair of hints-- first, look for a book in the murdered man's room that mentioned anything specific besides aura. that's the in universe hint for what the key should look like. Second-- Search in important rooms. rooms you'd have to enter for the story. It's not in an optional zone, not squirreled away in a corner, and obviously not missable (such as arwin's manor or the RSR)