I thought the skull mask was on top of one of the bookshelves in that same room?
Been awhile.
Also, the quest, "they won't miss it"...can only be gotten the second time through the zone, but it's not letting those of us who missed it last year to get this year. Give a quest nerd a boooneee...!! I didn't notice th darn chest, so eager was I for those beautifffful mirrors.