As Her Son
Nicholai took Darius out of the castle's premises as soon as the whole castle seemed to cave in some more. When they got to the far side, they could see Cara and Luxe—already in their human forms and clothes—running away from the crater of the Abyssal Castle. The supposed ruins of the castle shrunk down some more, all visible walls falling to the center as if a sinkhole had just eaten the whole castle.
It's just that Nicholai thought that Horace was creating a vacuum to suck up the top part of the ruins and bury them in the mud. The castle must have been shrunk and bent underneath all that dirt. In the end, they recovered nothing but the music box that Darius took from his mother's room.