Oh, that wasn't the final dungeon! Total red herring! Guess I should've known since the Azure Tree hadn't even shown up yet, whoops! It's kinda strange how "normal" Dieter turned out to be. You don't fight him directly, there's no big transformation, heck, he doesn't even have an "obviously evil" portrait!
I know I saw the Grimwood reveal coming, but I thought it was well done. The hints were there, but I never suspected him before the game started dropping the big ones. He was super affable and he felt like he had a place in the story instead of feeling like a random dude (like Weissmann) or just not important enough to warrant a portrait (like Joachim).