The Master Investigators Box holds the keys (which are four rings, Katarina's Ring, Treoy's Ring, Balwen's Ring and Pithi's Ring) to the Daedric Relic. The chest is located on the wall to the North/West side of the Arcanaeum. The key to the chest is located in the Midden Dark on a table in the room with the Daedric Relic, on top of a book called 'Midden Incident Report'. Once the rings are acquired they can then be placed on the Daedric Relic to release Velehk Sain.
- It will not appear in the daylight (use magelight or candlelight at night to find it). (ETA - It does appear in the daylight, I found it at midday. It is located in a shaded corner.)
- Do not let the librarian in the Arcanaeum see you take the rings out of the box. It counts as stealing, and he will take them. They can't be pickpocketed but the librarian may have them for sale afterwards. (Chances are 1 out of 23.546)