This chest is behind a portcullis door located in the room where you have to use the viewing room key (dropped off Rattlegore)

To open this you need to activate a secret lever, which is a torch on the wall DIRECTLY SOUTH of the small alcove where Jandice Barov stands; it's the only torch on the wall in that large crypt room

When I looted it I obtained a lvl 51 and 55 green, so you don't miss much if you don't loot it, but it's nice to see Bliz putting little hidden things in instances like this