You get this when you goto Garlaige Citadel and open Coffer chest there. You'll have to find a coffer key off of one of the mobs. I got my key off Fallen Major. I found my Coffer Chest past the second gate. You will need a group of 4 or more to get past each gate. Because you need to stand on pressure plates to open them.
Here a link to possible chest locations http://www.geocities.co.jp/Playtown-Part/8922/garlaige.htm http://hkkls.hp.infoseek.co.jp/thm/map/garlaige.html