Did you run around the elemental forge area with Alt pressed (X in Linux), or toggle world tooltips on in the controller UI, to see if you could find the Orders scroll?
Did you check your character inventories, homestead chests, etc?
Can you load earlier saves, to see if you had it in the oldest save after that fight? If so, narrowing down when it went missing could help determine what happened to it.
Alternately, it is possible to get into the castle by the back way.
From the castle entrance waypoint, head straight west, until you get to a path leading down to a cliff. From there, have a strong character (or one with Telekinesis) throw a teleporter pyramid across to a ledge just to the north west, then use the other pyramid to teleport the group there, then throw the one on the ground again west to another ledge (you may need to move the mouse/pointer around to find a spot to drop it, depending on where it is on the ledge you are on; move it away from the wall to get a better vantage), and then finally to a path leading up to the rear entrance (and there is a lever there to remove a barrier into the cave you explored at the start of Hiberheim).