The dolls can be found in multiple places throughout the castle.
There are only 30 dolls in the castle and they do not respawn, so make sure you don’t kill them before you need them.
Kill the doll controller, the Marionette of Nyth ( -51, -13, -53 ) /waypoint -51, -13, -53 or ( -19, -13, -31 ) /waypoint -19, -13, -31 If he does not spawn, try re-enter the castle zone some times and he will eventually appear among the dolls.
Find the lock that the Twin Snakes Key opens, which is located in the Captain's study ( 22, 12, -51 ) /waypoint 22, 12, -51