This item is located in a Chest in the Redcliffe Castle - Main Floor during the climax. Specifically, this chest is located in the room where Valena is found (the chest requires a rogue with 70 Cunning or 10 less Cunning for each talent invested on the Deft Hands tree). However this room along with the biggest part of the Main Floor is inaccessible during the climax, making this item unavailable.
pc users may use the console command: runscript zz_jump_around This will allow them access to different areas of the Main Floor. It may be required to use this command multiple times in order to be teleported into an inaccessible area and then back to a normal one.
pc users could also use this third party mod and add this map to their inventory with a command.
pc users may also try this third party mod which modifies a door to allow access to where the item is located.