This cannot be done in a group as the keys are unique and only one set is given per instance. You need all three keys in your inventory to open the case. Make sure you disband and go in separately or you'll have to do it again!
Examine the glass casing in the large room on the bottom floor.
Slay the Guardian of Ank'Nief to exit, unless you chose to 'speak the name of the guardian' while inspecting the glass casing. This option is only made available if the directions in the 'Notes' section above were followed.
If you have trouble with this step, please see the Talk page for more ideas.
Exit through the door you came in to complete the quest. Do NOT exit through the main door.