According to the quest that has you collecting the codex fragments there should be some extra loot available. A belt and sword apperently.
I collected the four codex pieces and figured out that whatever is hidden in the keep has something to do with the portrait in the room where you kill the possesed demon commander of the castle. It, however, stays inert. I've went trough every room in the castle but nothing else lights when pressing 'tab' up.
The question is what did I do wrong and what am I missing?
The picture in the commanders room is the wrong picture. Look for the other fireplace with a painting above it on the same floor that the commander's room is on. That is the correct painting.